Silicone baking mats. Relatively inexpensive, pretty strong, and the plastic didn't stick at all. Note, however, it's got a texture to it, so the skates didn't turn out crystal clear like I wanted. If you wanted crystal clear, you'd either need something very smooth as the form, or you'd have to do a lot of work after the fact (grinding, buffing, polishing) to get it clear. I've also read about people paining a very thin and smooth layer of plastic on after the fact, but I didn't get around to trying that.   (20/39)