I am in the process of installing a customized shower in a special purpose trailer where weight is critical and space is both limited and odd shaped. I expect to have a custom shower pan made but beyond that I'm uncertain. Since no standard enclosure will work I need to build something myself. My thinking is to use the Kerdi system (walls are 5mm ply) but I don't know how much weight per foot the mortar will add. Even more important is what kind of lightweight finish surface I should use. I don't like plastic tile but I don't really know of an equivalent (weight-wise) alternative and I don't know if it is compatible with the Kerdi Membrane. Any suggestions would be greatly appreciated.