Well the plastic I like to use is the stuff you get from MicroMark catalog, pictured below. I've played with recycled stuff, ie from kids toys there is usually many flat pieces you can use. The best stuff that doesn't give too thick a canopy is the K&S stuff in .015 thickness. When I cut it into uniform size to fit the frame I end up with @ 6 pieces per sheet and I calculated it out to @ 25 cents a canopy. I bought a Huge sheet of PVC clear at .020 thickness 4"x8" for @ 20.00 and it comes out to @ 6-8 cents a canopy. The .020 can be used but you have to heat it dangerously close to meltdown to get a great scale thickness. This is what I used for the finished P-40 canopy that I'll show you later. It is just a tad too thick but acceptable. I prefer the .015 but no-one makes a 4x8 sheet here in the US. I'll try to find all the spares I have in various plastics to show some of the possible outcomes.