"a wild theory as to how it might be done?"

I've never done this but here's a wild idea. Make up a flat sheet of several layers of glass cloth. Cut a ring with an inside diameter of 5" and outside 6". Epoxy this behind the hole as if you were putting in a hatch. Cut a 5 1/2" circle from the sheet, drop it in place over the hole, epxoy in place, fill fair and sand. Voila! Or you could put in a small day hatch. Good luck.