What's a good product to use to repair/replace the usual broken areas on an E body heater box?

Resto Rick (www.restorick.com) posted a very nice heater box repair process that he uses, you should check with him, VERY impressive work!

I bought the rebuilt kit from RestoRick and it came out awesome.

I think I spent about $40 for the kit that contains all the gaskets and foam for a rebuild.

For the fiberglas housing, I use the stick marine epoxy. Works great.
Just form what you need and sand if to shape.

The housing is not fiberglass, it's a composite injection molded plastic part. You need to see what resto Rick did to repair his, I've never seen such a nice looking (and original appearing) repair. In fact the same repair could be used to fix an under hood air box for an air grabber car (same material as a heater box).

Heater Box Repair

Rick