Building a sunken tiled bath
I once built a bath floor with white cement, and then covered it with sikatop-107 seal. it was an outdoor shower but it will fit also for an indoor one. here is a link to an article i wrote about it in my website. sorry, it's in Hebrew - but you still can look at the photos :)

you can build a structure out of salvage materials, plaster a thick layer of cement on it, and finish it with the sikatop-107 seal.