You could use a paint-on surface waterproofer such as RedGard, or a surface membrane such as Kerdi, or Hydroban, but then you'd need to cover it with tile. How are your going to hide the pan liner if you don't put something up on the concrete block wall? You need your waterproof layer to be sloped, and that isn't the tile...it's some kind of membrane. Kerdi or Hydroban could be applied directly to all of the walls, and could also become your liner, then tiled. If you use Kerdi, you can build your other walls out of regular drywall, since the membrane is totally waterproof.