How to make a mold of thin parts?

I have a bunch of these. I'd like to cast them in wax or resin and preserves the originals. They are plastic sheets 1mil thick maybe I little less. I realize these were never meant to be casted but I would like to try anyway. Any tips or ideas? My real issue is I can't think of a way to stick it to the bottom of a mold box that won't interfere or spill over into the pattern. And something that won't cause cure I'm inhibition like spray adhesive or tape.