If it is a normal skin fitting hole it should only be around 2" in diameter. I would taper it just enough to get several layers of glass in there.
Personally I would taper each side only about an inch so 2" becomes 3" diameter. Then epoxy
glue a 10 mm thick disc of foam in the middle. Then lay up increasingly larger diameter discs of double diagonal glass onto the bit of foam that is in the middle. Do one side then the other Fill the last bit on each side with an epoxy
filler mix to enable it to be sanded smooth.
Work the epoxy resin mix into the glass with small 1" or so disposable brushes
. Double diagonal glass cloth is very strong and has sufficient bulk to fill the hole. I wouldn't use chopped stand mat as it has little strength being a bit like blotting paper.
I've done repairs
in a similar fashion and the hardest part is obtaining a small piece of boatbuilding foam. A friendly boatbuilder
might have an off cut.
That's going to be a lot stronger than the hole originally was.