First step is to trace the sides onto the back of a single FRP panel using our trusty ceiling-curve template.

Nice fit. The gap at the back doesn't worry us because we'll build the back wall out a bit using leftover OSB.

To cut the FRP we used a sabre saw with a metal cutting (fine-toothed) blade. We backed the FRP with a sacrificial piece of plywood or OSB...whichever was handy and fit the need. Otherwise the FRP is entirely too floppy.

Looking forward.

Looking back.

Panel of 5/8" OSB to bring the back even with the sides. After this photo was taken we added 2 6" wide pirces of OSB in the ceiling curve for more anchoring surface area.

A little work with a sander and the rear FRP panel fits pretty well. There aren't any gaps that can't be caulked for water-tightness.
We'll finish the front of the tub with a panel of plywood on a hinge so the plumbing can be accessed if needed.