for backing to reinstall the hole, cut short popsicle sticks and glue them to the inside of the hull to make a "shelf" for the cutout to sit against. Use cloths pins clips to hold the sticks to the hull while the glue dries (epoxy or polyester resin) and be careful not to get any resin on the sides of the opening so your cut-out will fit back in without issue. Then mix up some more thickened resin and put it to the sticks and the edges of the opening (lightly). fit the cut-out back into the hull and rotate it back and forth to squeeze out the resin and get it at the same level as the outer surface of the hull. Tape it into position and let it firm up. With a small ziplock bag, fill it again with more thickened resin and just nip off a corner of the bag with scissors or a razor blade. Use this to squeeze in some more thickened resin around the perimeter of the repair filling the gap between the hull and the plate like icing a cake. This resin should be the thickest of them all - nearly like peanut butter in consistency. Scrape away any excess and let this cure. Then, with a side-angle grinder and a flapper sanding disk, I would sand around the perimeter of the repair you just made and dig into the fresh resin creating a little dished out area to lay in some new fiberglass to join the outer skin back together. Cut a bunch of fiberglass pieces and start to build the shallow U-notch you made back out slightly beyond the outer surface of the hull. Let it cure, sand with palm sander, skim coat with some premium bondo, sand, fill pin holes, gelcoat.