I am making a custom shower, planning to cut my own stone slabs from stones found in my area. They will be granite, quartz, some others, that I cut with my own saw. I'm wondering how to attach the stones to backerboard so that the assembly is waterproof. The pan will be thin concrete over backerboard as well. Considering attaching expanded metal to backerboard with a 1-2mm gap and then using standard thinset buttered to back of stone then setting to wall. But then how to waterproof the assembly?