Sometimes you can use thin blue foam to hand lay up fiberglass on.
You can tape the foam to the back side and feather the edge on the existing part you want to match. The layup goes over this and will later be sanded smooth and flat to match the surface. For greater strength you then fiberglass the back and you have a stiff sandwich composite made in place.
I scarfed some of the internal fiberglass walls removed from my Scamp
to fill in the many many cutouts in the camper. In this case I used heavy aluminum tape to hold the patched i place with the inside scarfed and layed up strips of fiberglass to bond them in place. The outside is relatively flush and I will use bondo to fair in the repair.
The above are examples of both sides of the patch behind the door where the big hole was. Sorry for the out of focus first shot, but you can get the idea.