Welcome, Coyote. How about a first name to use?
Well, I gotta admit, I'd give the same advice even if your customer's weren't nice.
You might look into the
Better Bench from the Innovis Company.
I think their small corner bench is too small for your purpose, but the larger one might work. I've used a shower with the little one and it's just not big enough for serious sitting on. I'm fixin' to install a large one in my current project, but won't get to test it for you.
You being a carpenter, I think I'd suggest you make them a nice removable bench/chair/seat of some kind outa some nice teak or Ipe or similar wood. Then they could just put it in the shower when and where it was needed.
My opinion; worth price charged.
Hehehe. Ol' Mikey typed so fast he done posted twict.