That stormtrooper mold is likely ultracal, hydrocal or some other sculptural plaster.
Clay will collapse under the pressure. Sculpey gets soft again when it heats up and will also be crushed.
You need something extremely strong.
For a sense of perspective, I used a spray paint can cap as a spacer on my table. It was crushed flat by the vacuum.
To answer the next common question from there... people usually sculpt the piece out of something like clay, then make a waste mold and pour the ultracal into that to make the forming positive.
Waste mold is a mold that you use once and throw away. I use alginate for that since it's cheap. Even being "cheap" it's still easy to spend $500 or so on waste mold materials on a big project.
Alternately, the positive can be made from something really strong in the first place. I do a lot of my tools out of MDF coated in bondo, with the details built up out of bondo and epoxy putty.
Either way, there's a lot of work to get to where you can form things