We custom built a shower pan for our '71 Tradewind. The base was made of layers of 3/8" ply, contoured like a topo map to slope towards the drain, with 1/8" lauan as the last wood surface. The front edge that gets the most bumping was made of a frame of 1x2 wood with a 1/8" skin on each surface. The curved part was made of two layers of 1/8" ply epoxied together and held in place with hands and elbows while it set (about 45 minutes - I'd do it differently next time, but couldn't re-design at the moment I realized that). The wall against the bulkhead was made of one piece of 1/4" plywood and has a gap at the top that allowed the pan to be removed for fiberglassing.
The whole thing was covered with glass cloth and epoxy, a coat of clear epoxy first to be sure of full penetration into the cloth and ease of bubble removal. Then the final epoxy coat was tinted white.
It's not perfect if you look up close, but it works like a charm and looks pretty decent. There are pictures on the blog: Tin Pickle Adventures: Shower Pan and Vanity