I also have not tried it, but was thinking something like silicone sealer, or maybe a type of glue that remains flexible when dry. There used to be a product called "Shoe Goo" that could be used in this way. You would want to test it first on something small, perhaps make a little bag, seal up all the seams and then when it's dry fill it with water and see what happens. Have now Googled - voila:
http://www.rei.com/product/679240/so...9-001b2166c2c0
Of course, by the time you buy enough of this kind of product plus your other materials you might have spent more than on a ready made bag, and your time is worth something.