I am working on a project constructing a basin for an outdoor shower. I am interested in constructing it with natural stone set in mortar. I am concerned about drainage and also the water affecting the mortar. I'm thinking of pouring a concrete drainage basin, then mortar setting stones on top. Does anyone have any advice on this? I'm also wondering how the mortar will stand up to the water that would flow on it with regular showering. Is there a recommended sealant?