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Hi Steven
I have done this many times. The answer is bondo. (that's what we call it here in Canada, its polyester based automotive body filler.)
Carefully in tomb the plasticine with bondo. Better tape off surrounding areas and wax over the tape. When you apply the bondo do it in layers, the first coat should be applied so as to capture the shape. Go past the shape to allow a flange around your piece about 2 inches. Once you have it about a inch thick you should have no trouble pulling it off.
Now dig out the plasticine and clean up any ragged edges.
So what you have is a rough mold. Wax many times and apply a number of coats of PVA. Apply a number of layers of bondo into your mold. I keep applying bondo in layers while there is still exotherm heat being given off. (just warm to the touch) Once filled to the desired thickness I pry the piece out of the mold while still warm. The reason I do this is that the bondo has not cured so hard that it will not break free from any little under cut craters, because this is a rough casting. The bondo is in a firm cheese state and will pop out easily.
Now you have a hard part that replaced the plasticene. Fit it to your part and you now you have a hard part that can be shaped into your mockup.
Hope this helps.
Fred
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