Top of the creature is sculpted, with respect to where the break of the two parts will be. I did have to slightly incise the Green Stuff a bit at the top to get it to separate but the join worked out well. Pink line indicates the break:
When I was sure I could not do any more damage to the sculpt, I began to clay-it-in for mold-making (it was my intention from the outset to try to cast a few in resin to make a little 'herd'). I began with a LEGO wall box, using a plasticine type clay to occupy form the barrier for the first half of the mold. Sprues and channels were made out of bits of wood.
The triangular part at the top is a pouring funnel built into the mold.