Spring Heater Box Cleaning - Damage Found

RRR's glass suggestion is good, and they also have the "Bondo glass" which has the fibers in the resin.
It's also easily sandable.

You could pick up a cow with the glass mat and resin, but the Bondo glass would only support a large dog.
The JB weld will work without a problem also, but may only support a medium dog, so a defroster hose would be just fine. :D