I used dense Styrofoam for my floor then applied a water proof liner with a thinset. It set my slope and was really easy to cut and place. I guess I glued it down to the plywood floor. The Styrofoam came from the manufacturer of my trench drain, The liner from Noble Seal along with the Noble sealant to make it all water tight around the edges. I have been using the shower for years and there is a bonus to having the Styrofoam it actually allows the tile floor to warm during the shower. Is it a selling point, probably not but it is quite noticeable during and after the shower when you have to step back in for some reason.