You may have just created the basis for dry-frame-bags.
I cut this bag to the size of two 2.5 liter soda bottles. I use it to carry 5 liters of water.
edit
Hm-mm; it took a few minutes for that statement to soak in. Would you modify a store bought dry bag to fit a bike frame? Is anyone up to date on some kind of Swiss technical fabric that
does truly keep water out?