I highly recommend you do NOT build anything on the fiberglass pan. Consider this; http://innoviscorp.com/better-bench There's many other options you can research, but be careful of some of the You Tube videos out there.
Come back when you've narrowed your choices to a few. Also let us know how you plan to waterproof the walls especially the lower part.
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.
MARBLE from ITALY & GREECE -PERLATO-PERLATINO-BOTTICINO-THASSOS-SIVEC-VOLAKAS-CALACATTA- NAXOS-Slabs-Tiles-Custom containers shipped to you - [email protected]