one liner i have considered, but not tried, is the fiberglass reinforced plastic shower liner (FRP) they sell at HD or lowes in 4 x 8 sheets. about 1/8 inch thick with a smooth pebbled white finish. finish vat framing, line with 3/4 ply then glue liner to inside. then i believe you could use a thickened fiberglass putty mix to fill and bond the seams like a fillet weld, and overlay the wet putty fillet with 6 inch fiberglass tape and paint in with more polyester boat resin. pretty much like making a stitch and glue boat. you could check with the company to make sure polyester resin bonds well to it. If so, it should work.
http://www.kemlite.com/structoglas/str_products.cfm
i would probably order some smooth finish sheets for better gasket sealing of pipe penetrations. the wood frame and ply take the water load, so reinforce the corners with angle iron plates. the liner just makes the durable water seal. I would also first paint the entire inside plywood surface with boat yard polyester resin, and let cure before applying the liner. extra strenght and moisture barrier to prevent rot.
has anyone tried this FRP shower liner?