I am using litho film that the students toss out (already exposed, developed and fixed), or like the package of 16x20 litho film I have that is too old to doing anything else with but fix it out. It is .004" thick. I used some that was .007" thick, but had trouble getting it to lay flat in the contact printing frame during exposure. I have better frames now, so it might work. I get many many uses out of a sheet. Recycle and reuse! (I transfer onto old photo paper, too!)

You should be able to re-use the Yupo, depending if you can pour on sheets that are already the size you want to print with. I know Sandy likes to pour large sheets and cut them down -- a very efficient way, but difficult to re-use the material.

But just about any material, plastic, paper, or otherwise, can work -- as long as it does not deform in hot water or falls apart in the same. Some have used wallpaper (unglued) or that stuff they use to wrap houses with before nailing on the siding (Tyvec?)