Note: The following concerns my '85 with side bath. Rear bath showers may have compound curves, and what follows would have no application.
Just one, but I am tempted to try it myself.
I saw some 4X8 sheets of white fiberglass at a fire/flood salvage store last week. The interesting thing was they were thin and flexible enough to go up the outside wall around the curve and over to the inside wall. The back and inside walls are square with the world, and would only have to be cut to shape and size.
One would have to clean/sand/remove the old solvent, but it would not be unreasonable to use those plastic mirror stars to hold it firmly in place, should adhesives not be enough.