dagger board case...

I had a Hobie 18 that had similar damage (I also had a 5.2 that I still regret selling). I would take some cardboard paper towel tubes and coat them with several layers of glass and epoxy. Once you have the damaged area pushed back out, I would put several layers of glass on the inside of the damaged area and use the tubes, trimmed to fit the sides and bottom, to brace the area to the upper sides of the hulls. It might be a good idea to put a brace between the two tubes forming sort of a v-shaped "H".