I don't know if this will be helpful or not...
I have been in the Thermoforming business as a maintenance manager for more years than I can remember.
For PET production tooling (60,000 lbs/day of product at 45mil) we use Syntactic Foam
for the "Plug" or male side of the formed piece.
It is smooth. available in many sizes, holds up to heat, is machinable, and can be finished to a very smooth, even glossy finish. You wont be able to work it with a hobby knife.Plastic doesn't stick to it. It is fairly hard and breaks when dropped.
It is kinda expensive
It can be glued together with epoxy and also comes in castable gallon liquid.