tossing around an air box design in the voids of my brain....ideally all alum. would be nice, but no TIG welder and it seems like using fiberglass would be easier (famous last words, hahahaha), however, for increased strength and for attachments, aluminum or even coated steel would be thrown into the design....

Check out "West System" epoxies at a marine supply place, they make numerous resins, and filler that you can blend to bond metals andglass/kevlar/carbon etc.

Please allow me to speak out of my own as I have no personal experience with fiberglass yet (but I've been researching). Anyhoo, for flanges and the like I think if the metal is prepped well (no oxidation or oils) then you can just embed it in the fiberglass during the layup. I've seen some fiberglass bumpers done that way.

What you're asking sounds more like when you want to bond a flare to a metal panel.

It's ok to have voids in your brain, but not in your fiberglass layup

Ok, that wasn't really lol funny

Many people have fiberglass fenders on their metal 914 fenders and I have alot of experience with boats and fiberglass. I always wondered if FG flares would eventually delaminate due to heat and cold conditions during winter due to different materials expanding and contracting during weather changes. Steel on steel is an easy no brainer and a buzz box with a tig scratch rig would be an easy installation. I'd put flares on a racer for big tires and rivet them on tastefully but for a street beast, it's gotta be pretty!