Get yourself two hose clamps that'll fit around the tube and some rubber strips to put under then
so they don't scratch the paint up too badly.
They'll hold a bottle cage on just fine.
If you can see under the dirt and grime, that's exactly how the bottle cage in this picture is mounted: Edit: Here's another picture of a bottle cage mounted with hose clamps (not mine).
bicycles.stackexchange.com/users/8/Goodbye Stack Exchange
The Twofish Quick Cage has worked well for me.
Not terribly pricey, and easy to reposition as needed.
Not as secure as the hose clamp solution, however.
bicycles.stackexchange.com/users/1273/Dan Blumenfeld
I know this isn't what you asked, but have you considered a Camelbak ?
bicycles.stackexchange.com/users/188/Mark Ingram
Are you only considering a frame mount?
If not, there are options for mounting a cage on the seat rails , seat post , and handlebars .
In addition, if the frame is all that you'll consider, there are a number of options for "clamp-type" water bottle systems .
You didn't mention, but if you have a carbon frame you'll need to choose an appropriate clamp so as not to damage the frame.
Not sure if you use a seat bag?
An option is to switch from a seat bag to a dual-mount seat-rail water bottle cage; and then, carry your extras in something like this, http://www.amazon.com/Tacx-Tool-Tube-Storage-Bottle/dp/B002SR0JRM and if you're frugal, a water bottle could be cut-down for the purpose.
bicycles.stackexchange.com/users/None/
I've been facing the same issue, trying to mount an additional bottle cage to the underside of the downtube on my mountain bike.
I haven't settled on one particular solution yet, but I found this site helpful as it outlines various different solutions: http://www.nordicgroup.us/cageboss/
bicycles.stackexchange.com/users/1166/Russ
As a Strida owner, I've struggled with exactly this problem.
I went with the KLICKFix solution, and couldn't be more pleased.
It's not always
I like to carry a bottle, so
I really like the fact that I just remove the whole thing in a split second.
bicycles.stackexchange.com/users/1119/T0TTE
Another option is to fit additional threaddd mounts to your frame.
This works fine if your frame is steel , but I wouldn't try this on a carbon or aluminium frame.
The threaded insert is called a rivnut or a nutsert and looks like this: You would drill two holes in your steel frame barely big enough for the OD of your nuts, press the nuts into place, and then use a rivnut tool or a suitably-threaded nut and bolt to crush the rivet part closed.
The thread is 5 x 0.8mm and is the same as your normal water bottle mount bolts, and coincidentally many wheel skewers too.
Epoxy or JB Weld would be a good idea too to help stabilise the insert.
You can also hire rivnut tools from hire companies - they're not cheap.
bicycles.stackexchange.com/users/19705/Criggie
On a carbon frame I used cheap Zefal Gizmo mounts for attaching kit the way it was intended.
Below you can see it carrying a Kryptonite Modulus lock on my bike
but you can obviously attach a bottle cage or whatever and wherever you like.
I did 100s of miles with it without any issue.
I'm not sure if the mounts came with rubber strips as you can see on the picture.
I may have used some spare ones from cycle light mounts for extra grip and protection.
You could cut out such strips from an old tube.
bicycles.stackexchange.com/users/13245/Johnny Baloney
I only have one set of water bottle mount screws on my bike.  I would like to have an additional water bottle.  Ideally I would like to mount the bottle on the frame (preferably on the vertical tube).  Bonus: Can I order/build in Canada?
Mount additional water bottle on bike frame with only one water bottle mount
bicycles.stackexchange.com
<hydration>
bicycles.stackexchange.com/questions/3358
bicycles.stackexchange.com/users/134/sixtyfootersdude
There are a number of solution for attaching another water bottle to the bike. Twofish Quick Cage and KLICKfix are two. Mounts for the fram can also be bought for attaching extra equipment. Moreover, mounts can also be attached to the seat, seat post and handlebars. It is possible to use a water bottle system based on clamps or attach thread mounts for additional support. Additionally, you could consider mounting a cage on your bike. There are a number of relevant products available.
There are a number of products available. It is possible to use a water bottle system based on clamps. You might want to attach thread mounts for additional support, for instance Zefal Gizmo mounts. You could consider mounting a cage on your bike. A good choice would be the Twofish Quick Cage.