I'm looking for some type of diffuser to place over a flash so I can use 35mm camera lenses wider than 35mm. I'm looking to use lenses from 28mm to 16mm fisheye.

The flashes in question are the Rollei E36 handle flash and an assortment of various shoe mounted flashes. Rollei never made (as far as I can tell) an add-on diffuser for the E36 handle flash. I did get one of those rectangular box-like diffusers in a box of stuff but it does not fit over any of my shoe mounted flashes.

How about this thing? It looks like I can strap it onto the E36 flash and get a good wide field: http://www.lumiquest.com/products/ultrabounce.htm

(link points to Lumiquest Ultrabounce)