Attaching a shower curtain rod to a fiberglass surround is problematic because the thin fiberglass does not allow the screw threads to grip. Tension rods offer one possible solution, but their styles are limited. Using toggle bolts provides a better solution.
Gather the hardware
There are many options for holding the shower curtain other than a straight, aluminum rod. Installing with toggle bolts allows the use of fancier shower curtain rods, such as curved rods. Obtain toggle bolts that are small enough to fit into the opening behind the surround when fully open.
Determine the location
Install the shower curtain rod approximately 2 inches higher than the length of the curtain. Mark one end for height. With the help of a friend, use a level on the rod to mark the other end. Put masking tape over the fiberglass where you plan to drill, and use the rod bracket to mark the location of the holes.
Drill the holes
Select the appropriate bit, and allow the drill to cut through the fiberglass without forcing it. Remove the masking tape.
Attach the rod
Assemble the rod. Insert the screws through the bracket, and start the toggles on the threads. Push the toggles through the holes, and tighten the screws. Do not over-tighten. Hang the new curtain using shower curtain holders.