First I softened plasticine in the microwave, then built it up around my model. I leveled it out right at midline, and I poured the first half of the mold in plaster.
After curing I removed the plasticine and poured the second half. After letting the mold halves dry for a couple of days I held them together with large rubber bands, and filled the cavity with the liquid neoprene. The plaster absorbed moisture from the rubber and a thin wall of solid material built up.