[Finster]

You can also use a dowel. Drill your hole until you find dry wood, coat the hole and soak the dowel with epoxy, and drive it in. Don't make it flush. Leave enough room for little cabosil and gelcoat finish.
The other thing you can do is drill an 1/8 inch hole above and on a 45 degree angle into the main hole, fill cabosil into the main hole til it comes out the small. Cover the main hole with something no stick, and top off the small hole. That way you won't have to worry about the air pockets you'll get if you just tried to jam cabosil in a horizontil hole.

Good luck.