Rainbow with Heart Plaster Mold 5-1/2 x 12-3/4 Inch. We carry the largest selection of plaster molds for sale on the web for plastercraft. Over 600 plaster of paris molds to choose from at great cheap prices. These plaster molds are made out of a durable plastic and can be used hundreds of times. Just mix up plaster and pour into the mold and let dry overnight. You are now ready to paint and decorate your plaster molds. This is a great inexpensive craft to buy for schools, camps, diy projects and birthday parties.