If you are looking for inexpensive, but durable, waterproof, and strong, I say go for corregated roofing framed out with wood.
If you use the metal kind, you can bend each sheet in the middle to create the corners, overlap them in the middle of each wall, just as you would on a roof, and ensure a watertight seam with caulk/construction adhesive in the overlap joint. For the fiberglass kind, just cut a 3" PVC pipe in half, put it in the corner so each wall is cradled in the curve, and caulk.
For the tub, use a mortor mixing bucket. I have a 2' x 3' that I hope to use for this purpose, but they do come a little larger than that. Look in the cement section of Lowes or your local big-box home improvement store.
You can be as cheap (3 shower curtains on rods), or as expensive as you can afford (completely tiled stall). Just up to your imagination and pocketbook!