[ocspray]

How would you attach a fiberglass part to the rest of the airbox properly?

I think it'd be easier to make a whole new airbox out of fiberglass (maybe that's what F-1 Si was suggesting). Mock up the box, complete with whatever inlet port(s) you choose, then make up the fiberglass parts. I'm guessing you'd want to use some kind of pre-made inserts for hose attachment points, and possibly mold a metal frame into the airbox for mounting points. Gaining access to service the filter might be tough, but making the top side flat with a surrounding flange for mounting a simple plate might be easiest to construct and seal.