Build a square frame for the volcano from two by four boards. Use half inch screws to hold the wood together. Screw four bed wheels to the base of the frame so that you can easily move the volcano across the stage. Place two or three boards across the middle of a frame to provide an area to place the volcano fire.
Staple chicken wire to the wood base in the desired shape of the volcano. Leave an opening large enough for a small fan to fit inside the volcano and blow air up through the opening.
Fill in the chicken wire with polystyrene pieces. You can use packing peanuts, blocks of polystyrene or any other form of polystyrene that you can find. Glue the pieces of polystyrene to the wire with fast-drying glue.
Dip the plaster strips into a bucket of warm water. Drape the plaster over the volcano base. Completely cover the entire volcano. Leave a hole in the middle for the flame and ash to sit. Allow the plaster to dry for 24 hours.
Paint the volcano with acrylic paints. Allow the paint to dry for two hours.
Cut a triangle of white silk about the same width as the volcano opening. Make the triangle twice as tall as the opening measurement. Fit the bulbs into the light fixtures. Place the lights inside the volcano so that the point at the opening.
Place a small fan inside the volcano. Position the fan so that the air blows upward. Staple the silk fabric to the sides of the volcano. Sprinkle the top of the fan with crumpled up paper to represent ash.
Test the volcano before using it on the stage. Turn on the lights. They should shine on the silk and look like fire. When you turn on the fan, the pieces of paper should fly out of the volcano and the silk piece should flicker like a fire. Make any final adjustments before using the volcano.