If you're worried about the crossbars or rails hitting the fiberglass and rubbing or breaking it, then you could try bolting some sort of padding to the roof. I'd recommend using round-baler belting. You can get it at most farm stores that supply tractor parts. Some sell it in pre-cut rolls as splicing pieces, others sell it by the foot. Its about 1/8" thick and 4 inches wide, though you can get 12" wide if you prefer. Its tough stuff and should do what you need.