I used to work in aluminium, I would use stainless rivets as the quickest way to build such a box. As I read your post I was thinking, since ambient air temps here reach 45C (and under bonnet temps must be approaching 80), I'd just rivet an alloy box together in about 1/2hour, and save the mess of fiberglass. All depends on what you're comfortable with I guess. You could also get powdercoated aluminium to reduce the risk of shorts.
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