Thanks for all the suggestions. I decided that glues and epoxies just weren't going to cut it in this situation so I decided a mechanical means of attachment was necessary. So I scrounged a piece of metal from the workbench and riveted it to the heater box and then riveted the bracket to the metal scrap. Looks like it'll work pretty slick. Some of the case clip mounting grooves are also broken out (someone probably replaced the fan motor and butchered the box because they didn't know how the clips work). So I think I'll use a similar idea and rivet some pipe strap the hold the halves together.
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