This is gonne sound stupid but heres how i did it...
Mask the entire outside with plastic wrap, get tubb of petroleum jelly, rub it down, get a few cans off tough stuff foam, spray all over the masked area, let it dry, dry coat the ♥♥♥♥ out of the foam (after removing it from the masked surface) with primer, get a can of plasticoat spray paint, spray paint it, next time you need a mock up slather it down with PJ and then fiberglass it and walla you got a "jig" if you will...