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