Building my DIY shower/bathroom, did I forget anything?

Take a look at a plastic basement laundry sink with the legs trimmed off as a ready-made shower base.

It gives you a 24x24 mini tub to stand in, plenty big enough for a shower. It's flexible plastic so it can take some abuse where fiberglass will just crack. A distinct advantage is it's deep to contain the water better than a shallow shower pan. Plus it has a lip that you can install the plastic paneling over, so no excessive gobbing of silicone.