I use clear vinyl a lot. I buy it at wal-mart. comes on a wide roll. and is sold by the yard. I get 1/2 yd and it is about 18" by maybe 50 " lots of stuff can be made with that much. I think that cost about $2. or less. Not expensive at all. For inside a bag I would get a shower curtain as it is lighter weight. What I get is more for in a wallet to put ID under or for using as a flap over make-up brushes.in a roll type bag etc. where you need something a little stiffer. Like someone else said I would use a heavy freezer baggie to try first.