Basically, you'd grind the three surfaces (front, back and top) to taper them so that you can build up a nice layered "patch" of fiberglass...
A typical taper is based on the thickness of the fiberglass involved, and tapers at 12:1.. so if the glass is 1/4" thick, the tapered section should be 3" wide. I would probably go with a bit more gradual a taper—16:1 or so and use four or five layers of glass on each side.
I would recommend you use epoxy resin, since it has the strongest secondary bonding characteristics (adhesive strength) and also generally has the strongest tensile strength. MAS, West and System Three all make good products for this.
To reduce the amount of fiberglass and resin that you'll need and use, most people will use a core of either plywood, foam or end-grain balsa. I'd recommend end-grain balsa or foam, since they're better and lighter than plywood as core materials go. If the coaming is thicker than 2-3", you may have to do the repair in a slightly different manner. Let me know how thick that aft transom section is.
The balsa or foam can be epoxied into the opening and then have the fiberglass and epoxy laid over both sides, with each layer overlapping the existing hull a bit more than the previous layer—say 1" each layer. Let the epoxy cure. Wash off amine blush if necessary. Sand it, fair it with thickened epoxy, filled with chopped fiberglass, and sand it to final shape... and then paint it.
West Systems, MAS and System Three all have a lot of good materials on their websites.