I had a 10lb can sitting around after fixing some cracks and decided to try it out. Well, it sets up very fast and on the mold I tested it on, it made a mold with great hardness and detail retention. I don't have any plastic to try out, but this stuff appears to be able to do whatever plaster can, only better in regard to casting. Much less messy. If anyone else out there gives it a try, please keep me up to speed.
BTW, it can be had in any hardware store.