The stalls that I've installed have usually been reinforced with a wood or thick fiberglass frame, some are foam filled.
Acrylic style shower stall with built in seat. What is behind the acrylic and fiberglass wall of the stall and under the built-in seast? Foam insulation? Dead space? Old newspapers?
Hippy is right K.B. I watched them redo my back bathroom shower stall and it was really reinforced with wood and acrylic fiberglass like material. Really strong and looks great !