After considerable research into different options for laying my floor I eventually settled on a straight up copy of what the guys at moreysintransit.com did.

Other options

I strongly considered two other options:

Key design elements

So what are the key design elements of the subfloor I decided to go with?

The total cost, weight and overall height is reasonable with relatively few skills required to achieve a good result. At this stage I haven't decided what my top floor will be (vinyl, carpet, hardwood?) or whether I will use metal or wooden carpet trim. Cheap OSB used as a template (scribed, cut, offered up & repeated until perfect)

Without the stock liner to use as a starting template I instead picked a cheap piece of 8'x4' OSB and scribed the wheel arch onto that. Through a repeated process of scribing, cutting & offering up the cut wood to the floor I eventually got a decent template. That said there were still some areas I'd overcut, so I marked these down on the board so I could make adjustments when transferring to the final ply. I bought three 8'x4' sheets of 15/32" ply ($28.45/sheet from Lowes) and had them cut the long edge to 69" leaving me three 4'x69" sheets plus offcuts to be used for filling the floor ribs. Transfer template to final ply; I then flipped the OSB and re-copied to the other side of the final ply for the other side of the floor

For the piece closest to the cab I scribed a few small areas directly onto the final ply, cut this and achieved a good fit. By overlaying my OSB template I was able to calculate the offset to start transferring my template to the final ply sheet.

Being new to all this I didn't manage to get everything sorted in a single day, but I finished up the evening by measuring the gaps between the floor ribs and transferring these to my offcuts for cutting the following day. The following day I finished off the top of the subfloor by cutting out the previously marked up lines for the middle sheet, as well as transferring my template OSB to the final sheet and trimming. The result is a good, tight fit which left just the curve of the rear entryway and the square 60/40 door step to cut which were done quickly.

At some point I will need to return to the short rear sheet in order to cut an access hole for lowering the spare tyre. Next up was securing the small offcuts to the floor of the van. For this I used 3M 4200 but unlike moreysintransit.com I found that a single large tube was not sufficient and will need to return to buy another to finish off the job. Prior to laying the sheets I cleaned the rear of the van with a brush and then 91% isopropyl alcohol to remove any grease and accumulated dirt - it had not looked this clean since the day I bought the van!

Currently the van is sat with the top floor reinstalled and all of my breeze blocks as well as other heavy items weighing down the floor to ensure a good bond. Once cured I labelled my work, installed a few final strips of Dynamat Xtreme (because why not) and began to prepare for the final installation. With the sub-subfloor complete I rolled out the underlay and cut around my final sub-floor template. This included an approx 2" gap at all sides so that the underlay would raise above the screwed down floor. The awesome 9" professional heavy duty scissors I bought made lightwork of the underlay.

Small cuts put at all corners in the underlay to ensure that it can fold up where it meets the sides of the van. I tried installing wooden dowels along the joints to keep the floor at a constant height but found I had insufficient space to get these to line up

With the underlay cut, I laid it down in the van and confirmed that the top subfloor still fitted snugly. In an attempt to keep the subfloor from lifting at the joins I drilled small holes along the edge and cut some dowels. Unfortunately in practice I didn't have enough space in the base of the van to align the two sheets and push them together.

With the subfloor laid I went around and drilled countersunk holes along the pre-marked pencil lines that lined up with the pieces of wood glued to the floor. In hindsight I should have drilled these outside of the van prior to installation to keep the sawdust from getting everywhere.

As I began to install the screws I found a few pieces of the glued wood to be insufficient for holding the screws. I believe this was down to insufficient purchase with the short screws (i.e. insufficiently countersunk holes to get the required depth) but also just because the quality of the top layer of ply varies along the sheet.

In a few cases I had to drill multiple holes to get a good secure screw. Currently this looks quite ugly but in future I plan to fill all of the countersunk holes with wood filler so the end result should still be clean.

On a final note there is one small corner of the van (in front of the driver side wheel arch) where the wood has warped slightly and does not sit flat. Unfortunately at this late stage I cannot see a way to resolve this issue so I will need to live with it. In practice I'll be installing a cabinet or similar over this area which I believe will keep the floor flat.
[null, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f4ddb29d6c9b31a2c0d/1491107672508/IMG_0223.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f4ddb29d6c9b31a2c0d/1491107672508/IMG_0223.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0223.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f4d8419c29a7b1d795c/1491107672539/IMG_0227.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f4d8419c29a7b1d795c/1491107672539/IMG_0227.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0227.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f54f5e2318397efb635/1491107675100/IMG_0228.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e07f4c03596ea5848dcbe4/58e07f54f5e2318397efb635/1491107675100/IMG_0228.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0228.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/t/58e0800d03596ea5848dd0ce/1491107901872/IMG_0238.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/t/58e0800d03596ea5848dd0ce/1491107901872/IMG_0238.jpg", "formatted_filename": "IMG", "alt_text": " Transfer template to final ply; I then flipped the OSB and re-copied to the other side of the final ply for the other side of the floor "}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bed4bebafbe50cc219d0/58e1bed4d1758e21442bdd4c/1491189469762/IMG_0241.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bed4bebafbe50cc219d0/58e1bed4d1758e21442bdd4c/1491189469762/IMG_0241.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0241.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bed4bebafbe50cc219d0/58e1bed4893fc0a495c014d9/1491189470168/IMG_0276.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bed4bebafbe50cc219d0/58e1bed4893fc0a495c014d9/1491189470168/IMG_0276.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0276.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf20893fc0a495c017d2/1491189547975/IMG_0267.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf20893fc0a495c017d2/1491189547975/IMG_0267.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0267.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf20b8a79b1475169ff3/1491189547438/IMG_0272.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf20b8a79b1475169ff3/1491189547438/IMG_0272.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0272.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf27d2b8572341206782/1491189551782/IMG_0273.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf1fcd0f6881b782a22a/58e1bf27d2b8572341206782/1491189551782/IMG_0273.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0273.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf508419c29a7b28085a/1491189594923/IMG_0279.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf508419c29a7b28085a/1491189594923/IMG_0279.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0279.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf50ff7c50123183b83c/1491189597167/IMG_0280.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf50ff7c50123183b83c/1491189597167/IMG_0280.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0280.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf57ebbd1a7ec649261c/1491189601924/IMG_0281.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf57ebbd1a7ec649261c/1491189601924/IMG_0281.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0281.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf5a44024364dbab4a52/1491189604839/IMG_0283.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e1bf50be65941eab16d6b3/58e1bf5a44024364dbab4a52/1491189604839/IMG_0283.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0283.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e676ebe58c625ba88e2c9e/58e676f020099e3168234113/1491498739855/IMG_0079.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e676ebe58c625ba88e2c9e/58e676f020099e3168234113/1491498739855/IMG_0079.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0079.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e676ebe58c625ba88e2c9e/58e676f0414fb51e9102f2cd/1491498739844/IMG_0080.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e676ebe58c625ba88e2c9e/58e676f0414fb51e9102f2cd/1491498739844/IMG_0080.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0080.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e677153e00be9c1e79a5c2/1491498777999/IMG_0077.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e677153e00be9c1e79a5c2/1491498777999/IMG_0077.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0077.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e677151b631b022cd303fa/1491498778008/IMG_0078.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e677151b631b022cd303fa/1491498778008/IMG_0078.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0078.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e67720c534a5168630f073/1491498790230/IMG_0081.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67714ebbd1a5be1432976/58e67720c534a5168630f073/1491498790230/IMG_0081.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0081.jpg"}, null, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e67749db29d6fe83fb568b/1491498831276/IMG_0083.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e67749db29d6fe83fb568b/1491498831276/IMG_0083.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0083.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e677493a04113f8da17c12/1491498830908/IMG_0094.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e677493a04113f8da17c12/1491498830908/IMG_0094.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0094.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e6774d6a496356f04c0316/1491498832791/IMG_0095.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e6774d6a496356f04c0316/1491498832791/IMG_0095.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0095.jpg"}, {"document_url": "https://lewiz.org/blog/2017/4/1/laying-the-floating-subfloor", "unformatted_src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e6774d03596e498e863ef1/1491498832861/IMG_0096.jpg", "src": "https://static1.squarespace.com/static/58d5bc0e20099e23c0d19bdb/58e67749a5790ab156018a68/58e6774d03596e498e863ef1/1491498832861/IMG_0096.jpg", "formatted_filename": "IMG", "alt_text": "IMG_0096.jpg"}, null]