Another way is to make a styrofoam(or balsa) plug, then cover it with a low temperature covering like Econokote. Next lay up a layer of fiberglass mat and/or add layers of fiberglass cloth over it as needed for strength.

You can use polyester or epoxy resin. Thin cloth (3/4 to 1.5 oz) is easier to work with than the heavier 6 oz cloth. The smoother you can get it on, the less sanding you will need to do.

When it's hardened, sand the bumps off with 80 grit paper. Try not to sand into the cloth too much. Brush on another coat of resin and sand with 100-180 grit. You can use automotive "spot putty" to fill rough spots. Prime with sandable primer and finish sand.

Fellows use different variations of this technique such as brushing hot paraffin wax over the plug, or coating the plug with water based polyurethane etc, adding wire for stiffness etc.

Some don't coat the plug at all and just dig the foam off or dissolve it with gasoline when done! If you don't coat the plug use only epoxy resin because polyester eats foam