Or perhaps try the beige wall foam with the foil face at Home Depot.  Peel off the foil, cut out "layers", glue them together (with spray can foam - Great Stuff) in a stack and sculpt the rough shape to exactly what you want.  When it is sanded smooth, wrap it with Saran Wrap, spray it with Pam, lay up BID over the mold and (if you like) wrap it again with Saran Wrap.
EZ to sculpt, EZ to lay up, EZ to dig out form .... Jim S.