Not trying to disappoint you, BUT that small heater core "box" ( behind the RH fender ) has been a challenge for me too. I don't need one at this point. I have several larger AC "boxes" saved up from several cars' but that little one over to the side is always damaged. I suppose due to its small size and fragile nature.
I've never tried fiberglass repair. Maybe that's a possibility. Otherwise I have no solutions to offer. Sorry.