I would not fill or do anything big with that hole. WHatever I do on my boat I try to keep it OD legal. I changed the main winches, but kept the old ones just in caseI want to sell the boat.
A thru hull fitting will not corrode away or get blisters, just put a plug into the opening and be done with it. No sense of making extra work...
just my 2 cents
Thor
