Rule of thumb: Polyester resin (Fiberglass) will melt Styrofoam. If you are going to use Styrofoam then use epoxy resin ($$). If you want to use the cheaper polyester resin you must use urethane foam. You can build up the dash with foam, cut it, or sand it to shape. Once your final shape is arrived at use the method above to cover it.
You could use Styrofoam and polyester resin but you must add a barrier to the surface of the foam fist to prevent the resin from touching the foam. This is usually done with clear packaging tape.