You made the mold into a cup. Why not just spray the inside of an identical cup with some spray vegetable oil and slip a new cup with the bottom cut off over the mold to hold it together? Also, you can pre-cast some small stones or "chalk sticks" to force into the plaster while it is hardening to fill it up instead of mixing more plaster when you run short. You have an incredible amount of voids due to air bubbles because you mixed it too thick.
Do you use mold release?
You made the mold into a cup. Why not just spray the inside of an identical cup with some spray vegetable oil and slip a new cup with the bottom cut off over the mold to hold it together? Also, you can pre-cast some small stones or "chalk sticks" to force into the plaster while it is hardening to fill it up instead of mixing more plaster when you run short. You have an incredible amount of voids due to air bubbles because you mixed it too thick.