I answered this question for you on rec.windsurfing, but I will take another shot at it here.
How you "fill up" the cavity left by removing the centerboard assembly is going to depend alot on how "permanent" you want your plug to be.
If you take a couple of thin sheets of plastic or fiberglass (already cured) you could cut
a fairly precision piece to fit in the recess where the rubber CB gasket was when you had the CB in the board.
If you want it permanent, you could glue it in with epoxy.
If you want it less permanent, you could glue it in with something like rubber cement or a latex base contact adhesive.
If you wish to fill in the entire cavity, you could cut a piece of hard closed cell foam (divynicell or something similar) and glue a piece into the cavity that would fill the entire cavity.
Then make a "plate" to go on the top that uses the same pattern and shape as the deck pad that held the CB assembly in the board.
Hope this helps,
Last edited by Roger; 19th March 2008 at 08:11 AM.