You've reminded me of something I'd forgotten to post. In reading prior to doing this job, I ran across an article where the author (a surveyor) pointed out that the reason for leakage over time was due to movement, by and large. Made me think that some sort of internal structural support would be a good idea to incorporate, and it could be aesthetic as well as functional - small shelf, a handhold/towel rack, etc... And I hadn't really thought of it in that way, but the "lip" in my first drawing would do the same, really stiffen up the structure, if constructed properly...