I've seen some new bathrooms where the whole space is tiled top to bottom, and the entire floor is drained to a space where the shower head and valves are placed. The shower is handheld so the entire cupboard becomes the shower . . Since a toilet requires 60" by code here I'm not sure if you can place yours at the end of the cupboard. . . lets say you aren't constrained by that code, and you take 30" at one end of the 80" lenth to place the commode. Now you have an area just larger than 4'2" x 4'6" to organize a shallow sink and the shower. If you use a bar sink and run a concrete counter just 12" deep all along the wall opposite the commode, and place the shower valves and heads opposite that about 15" from the other end ? If you do not enclose the shower, use cement board for the entire room, tile top to bottom and can do a subfloor drain - even a pair of trench drains / linear drains across the floor and along the doorway itself, this could work. The door may need to be a rolling door or pocket door . use an exterior grade wood and waterlox the door itself. This would work best with a window but barring that, install a very good exhaust fan and run it so moisture will be removed. . .