Spring Heater Box Cleaning - Damage Found

Me-thinks a prior guy was prying w/ a screwdriver to remove a duct. I fixed the cracked box in my 65 Newport, using "plastic epoxy" over fiberglass wall-board tape for tensile strength (Home Depot). I don't think the box is traditional fiberglass - epoxy. I suspect it is more like plastic extruded into fiberglass. I would test a section first to see if epoxy resin sticks. TrailBeast's suggestion to use Bondo glass sounds best. I have an opened can, so wonder why I didn't. I think the resin is similar to what is used w/ fiberglass. I recall they use the same hardener.