Consider sculpting in pink (Owens Corning) or blue (Dow Styrofoam(TM)) insulation foam, making an intermediate plaster waste mold, and casting a sturdy buck in that.
Don't vacuum form directly over pink or blue foam. You won't crush it, but you'll scrunch any protrusions stretching plastic across it.
There's a couple of relevant threads about this going on over at tk560.com.
There's another thread about hot wire cutters & jigs for sculpting foam, in the vacuum forming forum.
For short run stuff, or an intermediate mold I wouldn't form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement.What would I use to cast the buck out of after making the waste mold from plaster? Really dense resin or some kind of liquid cement hardner?