ORIGINAL: ...why couldn't i just use J-B Weld Epoxy?
I think that'll work just fine. I've used JB Weld to reshape metal parts and haven't had any problems and I've used various epoxies on fiberglass (including polyester-based glass) with excellent results. Clean it all well (I use acetone, MEK, or automotive degreaser (prep-sol) but even detergent washing followed with alcohol should be fine) and sand the gloss off... you'll be good to go.