I had to do this on Blue Note when I first got her; there were areas in the keel that had to be fixed because the PO had grounded her, and fixed the damage with Bondo. So the keel leaked, and there was water inside which had caused the foam to come apart.
I drilled a few 1.5" holes in the keel and sanded all the area around them bare. After letting them dry for a few weeks, I used a piece of glass and Peel-Ply to make up a laminate sandwich of 10oz cloth and epoxy, squeegeeing on additional layers as I went. It took about thirty plies of cloth wet with epoxy to make a 3/4" thickness piece about six inches square or so. When that was green hard (not sticky but will still dent with your thumbnail), I used a hole saw to cut out the plugs I needed, and buttered them up with thickened epoxy and silica powder. Then I used a plastic hammer to tap them into the holes I'd drilled in the keel where the bad areas were. Later on, I sanded them flat and epoxied about three or four layers of cloth over them in progressively bigger ovals until I got tired of doing it. Then I sanded the whole thing a bit and rolled bottom paint over it. That repair has held for years, as AFAIK is still in there.
There's an easier way to do this, now- you can get a scrap piece of FRP out of another hull, or a piece of premade FRP structural from companies like Strongwell, and just cut a piece the size you need out with a hole saw. Preformed FRP structurals come in all shapes and sizes and strengths, and can be really useful for repairs. The only downside is you have to buy a lot more than you need; they are often big sheets.
