Take a trip to a Plumbing Supply House and ask for epoxy to repair a fiberglass tub. And while you're there, you may want to inquire on how well it will hold up and if there are any 'tricks' in making the repair.
My best to ya and hope this helps.
Jay J -Moderator
PS: (The supply houses are in the YELLOW PAGES under PLUMBING - SUPPLIES ...)