How to Seal fiberglass Hole under waterline

I have a 91 ski brendella that I am repairing the transom. It had a rear drain plug at the back of the boat in the bottom of the transom. I have never used this (always used the center drain plug). It was very difficult to work the transom in from the inside and very hard to get it to the very bottom (which is where the drain plug is). I don't want to put the drain plug in because it is is so close to the bottom of the transom, I won't be able to get good strength around it where it screws in (even if the bottom screws were to grip the bottom of the transom).
I was thinking about just fiberglassing this hole in permanently but I'm not real familar with techniques on outside hull repair (especially underwaterline).
Do I just take a grinder and feather out the sides of the hole and then start laying fiberglass layers? If so, how much do I feather? What type of fiberglass mat/resin? how many layers? and how do I finish it? It doesn't have to look perfect because it won't be seen... I am mainly interested in permanent bond/seal/structure. The existing rear fiberglass is approx 1/4" thick.
Any fiberglass tips would be helpful.

 

If 1/4" is all you have for depth, you can dish in with the grinder, feathering so that the outer hole is about 2" around, shallow bowl shaped, and build laters of mat and polyester resin until even with the surface.
Then sand a bit below flat, tapering out another inch. Tape around the area about 4x4 inches. Add another coat over that, resin only. Place some clear poly plasic over that and squegee out until flat. When the resin cures, remove the plasic and tape and prime/paint.

Alan