Another solution is to use Styrofoam/Ploystyrene small or large cell blue
foam, readily available from Aircraft Spruce:

       1. Cut it with a hot wire or sand to shape (sands easily with another
piece of itself),
       2.  Coat final shape with a micro slurry before laying up part.
       3.  Lay up part.
       4.  Dissolve blue foam plug with gasoline.
       5.  Sand inside (micro) to a very nice smooth finish.

John Kleber
N83JK   LEG2/G