I would agree with the epoxy. I'd use layers of plywood glued in with a thickened epoxy. Square the hole first. As for the top finish you MAY be able to blend it in, but I think you would always have the danger of water intrusion. So, I would put a piece of sheet metal on there with serious caulking to make it water tight. In fact, you can put a sheet of aluminum to match all the way across and make it look intentional......