Or you could make a nice plug out of surfboard foam.
This stuff is great fun to work with. It cuts and sand super easy, is strong enough to actually mount gauges and stuff first, and does not melt from the fibrglass.
Then you can get a real nice tight fit to your cage, and do sweet complex and compound curves.
Once you get the shape you want you just glass over it for you negative, add a few scraps of wood to keep it flat and from twisting, and then wax it up and you have a good mold.
Now just lay in your favorite glass or carbon fiber. Then you can finish as you want with paint, clear, or vynil/ leather.
FUN!
Brad