Many plastic stores will have NON-ito-coated PET sheets in various thicknesses; much cheaper to experiment with!
PET == Mylar == Polyester film; the same stuff as most transparencies, for instance. (except SOME are "acetate")http://www.dunmore.com/technical/pet-film.html
(OTOH, differences in thickness and production techniques can drastically change the way a plastic film behaves. I think "Shrinky-dink film" is also PET, but clearly not all mylar behaves that way. (and ever heat up an empty soda bottle (also PET) in an oven?))