Half mold with modeling clay.
Working box made from vinyl sign material, not quite styrene. Built up the bottom with modeling clay to halfway. The plan is next to apply release agent, then fill with 2-part rubber mold material. When that dries, I'd reopen the box, clear away the modeling clay, flip over, reapply release agent, and fill with material again for the other half of the mold.
If you see me about to make a big mistake, please say so! This will be my first time through this process.