I am planning a 6'x4' shower on an exterior wall so I would rather not use a niche for bottles of shampoo and soap. I saw Schluter has a tile-supported stainless steel shelf, but I'd like a longer one, maybe up to 6' long to go across the back wall of the shower. Does anyone know how to build a shelf like that? Or should I just use a niche in the side wall which is interior? Thanks!