Mike, the original article by Dave on how to do this used to on the message board somewhere but I cant find it.
Basically, the hull and rudder is built up so that the gudgeons do not stand proud. The closed cell foam is used to build up and also to make the aperture smaller. After the foam is shaped to the desired shape, class cloth is epoxied over the whole thing to give it some strength. The hull and rudder remain unchanged under the foam.
What is eliminated is any forward facing obstruction that will cause resistance to the water.