This is the broken area. You can also see another missing piece where the exhaust fell off and broke through. I repaired that on the underside only for now and will deal with the external body side when I do the exterior prep. My approach to fixing this involved pressing the wet fiberglass between two pieces of plywood covered with wax paper.
I made a form for the inner support. After cutting it out I covered the edge and one face with wax paper.
I sanded and tapered the repair area. Once I mixed the epoxy and began wetting the mat, I didn't stop to take any pictures. Basically, I started laying the mat on the roughened inner area shown at the left. I built it towards the rear until about an inch was sticking past the flange. Then I used spring clamps to hold the form I made against the inner surface of the flange and folded the mat up against it. I kept building the mat onto the form until the required space was filled out.
Once I had enough material to form the repair, I put the second piece of plywood in place and squeezed them together. I used a paint stirrer to keep the plywood separated on the outer edge.
After the epoxy cured, I removed the press.
I used the other side to make a stencil for cutting the fiberglass. I made sure the stencil contained two holes so that I could get the third hole properly marked. I'm using the single piece bump[er cover retainers so all the holes must line up.
Clamp the stencil in place, use a sharpie to trace the outline, and cut away the excess with a dremel.
Here's the repair after drum sanding and drilling.
SUMMARY: Though the repair was easy to execute, I had to come up with what was, to me, a novel approach. I'd give this a 3 out of 10.
Early in this project, one of the first things I did was remove the rear bumper cover and bumper. While in the process of removing the cover, I realized that the flange had been broken at some point in the past. It was not just a matter of gluing it back on as it was shattered and pieces were missing. The walk through below details how I repaired that area.
Rear Bumper Cover Flange