I take back what I said about adding anything to Plaster of Paris. That stuff makes great molds without use of anything else, just a box to hold it while it firms up. Just spray a releasing agent, like cooking oil, so it will release from the mold. Plaster of Paris gets hard fast, so grease up your part ahead of time, make sure there are no bubbles, and stick it in the plaster, just as it starts to firm, better early than late, but not too early, or ou will never get it out!! Some strategically placed holes, a hair dryer blower, could make an excellent vacuum forming mold.
