Danno,
To fill the hole like you have made, I have made a backing plate for a fiberglass patch to go INSIDE the hole. I haver done this successfully by finding softer plastic material (like the side of a PVC bleach bottle with about the right curve. I put 2 small holes in the middle 3-5 inches apart and run a copper wire or heavy string thru the holes to the outside, tie into loops:
This patch backing can be bigger and longer than the hole because it is inserted sideways then twisted to fit the hole. Once you have tried it and are confident that you can get it in, pulled up and fits well, then you pull on the strings to pull your patch backing into place. Hold it there with a "strongback"- another piece of wood on the outside, placed across the hole lengthwise thru the loops.

The sanding is all done on the INSIDE edges of the hole, right?
Cut fiberglass cloth to fit (big enough to overlap the hole edges inside). Place 2 or 3 layers of fiberglass cloth on the backing plate (thread the string thru the cloth). Now use epoxy thickened with fumed silica dust or microballoons -& smear some inside the hole. Now wet out the cloth, turn the patch to insert into the hole, position it and pull up into place. Tie wires or string over the "strongback" to hold it up tight (engage wife for enough hands after being sure she has gloves on too)- and stick a wedge under the strongback to get it all JUST tight enough.

If you make a mess of it by dropping the patch or getting it all crooked, have another patch assembly ready to use. With epoxy you have enough time.

When this is all pretty hard the next day, it will fill the hole but allow you to sand a little (cut the wire or string & pull out). Fill by adding cloth pieces to the top of the hole, and overlapping the outside = A VERY strong repair.
Leave the PVC backing inside.