Monokote works great, just wax with a couple of coats of car wax first. Alternatively, if you only need a single part, you can make the plug from foam then glass over that and then remove the foam. Most types of foam can be dissolved with gasoline or you can just chisel the foam out.

FYI - Technically speaking what you are making is a plug. A mold would be the female counterpart to your plug.