I would take a pair of gloves (non porous) fill the gloves with styrofoam beads and then tape off the open end - mold your monster shaped hands in clay onto the gloves and then make a mold of your design. Cast the design and then either get a second pair of gloves if you want to keep your original sculpt - or remove the clay from the first ones and have then casted in with the final mold - after removing the completed monster hand /finished paint work etc... remove tape off glove dump out/vacuum styrofoam beads....and then it should.
Fit like a glove!