I know this is an old Topic but needs to be updated. First on a Fiberglass Shower wall you do not want to put any load on it as it will flex, crack or possibly break. Fiberglass is only so strong and when you fall the physics that take place is more than a 300 lb weight just hanging on a bar. Toggle bolts are only as strong as the fiberglass wall, the other mentioned mounts again are only as strong as the fiberglass wall. Remember fiberglass walls flex so you might have a bar attached to the wall but is it solid? No. The only kit out there is The Solid Mount, it was designed in 1995, was and to date still is the only Mount designed specifically for fiberglass showers and carried a U.S. Patent. The SolidMount is a mount that attaches through the front of the fiberglass and mounts directly to the studs of your home removing the load on the fiberglass and making your shower safe with a SOLID mounting for your grab bar. For more information go to The SolidMount Website.
Good Luck be safe and do not do a short cut it is your safety we are talking about.