I've thought about doing that in my old VA-108 wheel pants.
Make a bulkhead out of cardboard or similar that will fit reasonably well.
Apply plastic or release wax to it.
Wrap it with a couple of layers of fiberglass that will extend 0.5 to 1" beyond the edges.
Rough up (coarse sandpaper) the 1" circumference inside the pant.
Wet the fiberglass with epoxy and install.
When dry, remove the cardboard bulkhead.
Finish the fiberglass until reasonably smooth.
Finn