Sharp edges and fiberglass tape.

You might try a vacuum bag. I made a rudder a couple of months ago this way
(for a Flying Junior dinghy that I own). I was taking a class on composites
(for aircraft, not boats, but the instructor didn't care - "An airfoil's an
airfoil..." he said.)
this last term and got to fool around with this kind of thing quite a bit.

It's pretty easy to do. I'll be glad to explain off-list (unless there's
interest from
others - email me.) The initial equipment is a little expensive, but not
TOO bad.