Go to Home Depot and get some casting plaster. You will need to heat
it to drive the moisture out. Not just the non-bound water, but also
the water that caused the plaster to set. This makes it weak. So you
might try putting the piece on top of some waxed paper , then brushing
plaster on it so you don't have air bubbles on the surface of your
piece. Then put some chicken wire over the plaster and add more
plaster. Let all that set. Then turn over and remove your pattern.
Heat to about 400 degrees F ( I am guessing at the temp ) and then puor
the metal while the mold is still hot.

You might also try buying some silicon rubber caulking compound while
you are at Home Depot. It ought to work for one or two pieces. They
make silicon rubber that is more heat resistant, but I am not sure
where you would find it.

Dan