My first post here, but I am following the forum for many years.
I have some experience in plywood
and fiberglass epoxy
and polyester (built 13 small boats, the biggest 7 meter small yacht)
The task to make the new panel or to modify the old one is not so complicate. Some additional pictures (the panel inside and the panel location) will help to understand better the task.
1. Use the panel as a male or female mold
to make a part of about 400 mm length
2. Cut the panel in the middle
3. Send the edges and 50 mm inside left and right with rough sending paper to allow good bonding
4. Fasten with pop rivets the new part from outside to get the precise length
5. Use wax or PVA for separation of the part
5. Fill the inside gap with glass and epoxy
6. Remove the part
Same procedure with the heightening.
Finishing in the end.
If it seems too complicate, any car body repair shop can do it for you