Shawn, I have never used plaster for a mold half but I have used 2 part urethane rubber. I used shore 80 and am very happy with the results. You can not sand or alter the mold after cure, but it remains very slightly flexible, so the component can be easily removed. Plaster is supposed to make a very nice mold, but I understand heat during cure is an issue. How much trouble was the mold to make?

Scott