Here is something I do for blind repairs that works well:

1. Take some wood strips and epoxy coat them all over. (for waterproofing) Then butter them with thickened epoxy.

2. After opening up your hole enough to get some fingers in there, slide the wood strips into the hole, and using your gloved fingers, smush them from behind (stick your fingers in the hole, grab the stick, and pull it forward) so they are covering the hole. The trick with the last strip is to put a screw in it about center before sliding it in, then use the screw to pull it forward. Oh- I forgot to mention- each time you put a strip in you need to run a drywall screw through the hull and through the strip inside the hull to hold it in place. The drywall screws need to be waxed with paste wax, candle wax, or any release agent to keep the epoxy from sticking to them.

3. Once the epoxy goo cures, remove the drywall screws. You now have a wood backer to your hole securely epoxied from the back. Now you can start laying up fiberglass first with woven roving, then with cloth on top of that until your repair is up to the surface. Oh, and I forgot that you need to grind away the first couple of layers of glass around the hole so that you can overlap the last few layers of your repair around the hole and in the end have a flush finish.

Good luck!

Malcolm