[Veselin Kolev]

Fiberglass gets its strength from the glass fibers that make it. If you drill a hole in a fiberglass box beam, the glass fibers are no longer continuous, so you compensate the beam's strength. If you want to have a continuous box beam with cuts in it, you have to get it custom woven by a company, and that is pricey. I would stick to using fiberglass welding glue, or if you must, only drill screw holes in the very ends of the beam. That way you retain as much of the beam's strenght as possible.