I did the same thing Harry is talking about - used "FRB" as the bulk insert and matted/resined it into place. FRB is the stuff they use in public restrooms as wall board, it typically has a glossy and textured side, and a flat smooth back side. Home Despot has it in 4 by 8 foot sheets.
My rig had a 'stop sign' screwed and siliconed to the top to cover the old roof vent hole when I bought it. No kidding. Because a full paint
job was in the plan, I left the smooth side of the FRB exposed on the outside to recieve paint
, and built up the inside with glass.
I cut an 'exact' filler piece for the hole. Then roughed-up the gloss side for adhesion. I taped the piece in place - flush, from the outside. Then, working from the back, I ran glass matting strips along all four sides, then matted over them and built the repair up to the roof's thickness. I filled the seams on the outside with marine epoxy filler, then used Bondo to blend, feather and hide the repair. The FRB took the primer and paint
just fine. One year later it's still good to go.