I used a 2-part resin to cast the mask from the silicon mold. I mixed the liquid resin, poured it into the mold, and let it cure over night. I poured 3 layers of liquid resin to strengthen the mask, letting each set over night.

And I wore a respirator while working with it, in a well-ventilated garage.

Demolding the mask was easy, and I still have a usable silicon mold.