I have used the plaster of paris technique before. Cover the outside of the canopy with painters take to protect it. Wax the inside of the canopy with carnuba wax; 3 or 4 coats. Build the plaster up in layers of about 1/4" at a time; that keeps the heat buildup to a minimum. Use gauze cut from a roll or 2 X 2 gauze squares to adhere one layer to the next. Just press the edges of the gauze squares into the wet layer. Don't bother to make the layer smooth on the inside. When dry the next layer will adhere nicely. I only build up the mold to about 1/2" to 3/4" thickness; no need to make it solid.