Repairing fiberglass garage door panel with large hole
Find a good section of door with the same pattern.
Spray release chemical on that section of door.  Build a box/frame with a back around this area.
Fill the box with expanding foam, so it extrudes against the door area.
Release the foam and box from the door.   You now have a mold.
Paint the mold with just enough epoxy to give it a hard shell (harder than normal foam).
Spray the foam and box with release agent.
Move this foam and box over to the damaged area and  attach it to the door.
Now you have a mold to do a fiberglas layup in the normal way.