What type of washcloth bar should I put inside a fiberglass shower?
I know of 2 methods to create a safe mount that will support the fiberglass


Wood blocking directly behind fiberglass.  



  
    This method relies on access to the other side of the wall, to place
    the blocking (1x4 is close to the spacing) such that it is against the
    fiberglass and secured by 2 studs.
  



Special self supporting brackets (Solid Mount™)



  
    These can be placed from inside the shower, but require you to anchor
    the rear of the mounting plate to the stud (in 2 places).  If the
    spacing is not optimum for your towel bar, a waterproof sub mount
    (spanning the 2 mounts, made of solid surface material, Swanstone or
    Corian) could give you a few inches.  Cantilevering any more than that
    would be unsafe.
  




Most fiberglass enclosures don't go to the ceiling, so a "single rung" towel bar could be stud mounted just above the fiberglass.