Here's what I did on a shower stall I installed in my basement. I knew I needed to support the base, but I was afraid to use mortar in case I didn't get the unit set right. I had a sack of Armstrong Self Floor Leveling, and I mixed that up and poured it in one side and the back that was open. I had to temporarily caulk the edges with some old towels, then a bit of clean up after the stuff had set, but it worked just fine. I'm sure we could see under the stall that there are cracks in the product, it wasn't designed to be used in a thick build up like I did, but it provided solid support under the shower floor.