This is what I'd do ( I had a similar situation - and this is what I did ). My rot only extended about 2" around the hole. But, if you can clean out the bad wood, this would work:
First, you must dig out the bad core. I used a set of Allen keys put into a drill chuck to get the horizontal mush out. I used a compressor and vacuum to blow/suck to clean it out as I went. Take your time and get the mush out. Then, let it dry. I put plastic on the bottom of the hole, held in place with Gorilla Tape. I put a computer "muffin" fan over the top to exhaust air out from the hole - taking moisture with it - for a few days. If you can, keep a hearing pad on or around the area to help with evaporation.
When I was ready to fill with epoxy, I re-taped the hole from the bottom with plastic and Gorilla Tape - make sure you get a really good bond or you'll have a mess ( for such a big hole, I might use a 1/4" of plywood covered with plastic, screwed into place ( do not go through the deck !) - use Mortite ( like a clay ) sandwiched between the ply and the underneath of the deck, around the whole thing to avoid the epoxy from dripping out.
Then, mix up epoxy and a lot of chopped fiberglass ( must have a good flowing consistency ). I used a syringe to put through smaller access holes, but you could just poor into the void to fill it up. This will self level so if the boat is not level, neither will your epoxy be ( try to level the area - use a level and adjust trailer as necessary ). Depending on the area to be filled, it could 'drink' a lot - so be ready to mix another batch with chopped glass.
This is a very fluid mix so be sure that your 1/4"/ plastic/ mortite/ screw-job is leak proof.
Once dry, you could drill through to make a new hole - or, you could use Gorilla tape around the circumference of the opening to create a 'wall', so the epoxy stays within the area you want to fill - so you don't fill the inspection plate hole. This is how I did it and the Gorilla Tape held perfectly, keeping the epoxy contained where I wanted it.
My repair is rock solid...
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