Jeff, no, the ABS/Acetone mix would not be the way to go. Heater boxes are made of RIM molded plastic (Reaction Injection Molding) it's much like a fiber reinforced epoxy that is cured under heat and pressure. It does not have the typical characteristics of injection molded resins.

Repairs can be made with two part epoxies, I like this stuff:

http://www.jbweld.com/products/highheat-epoxy-putty

Larger patches can be reinforced with a patch of fiberglass window screen. After smoothing it over you can sand and paint to blend.

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