On first read, the thing that I thought of was the step needed feet on the forward side to interlock into the shower grating. This would keep it from sliding around while heeled.
That lead me to think of building something open like the shower grate. Too bad that I'm not competent with a dado.
But you would surely want something with an open feel to it so that the dampness it will get if let in place during a shower will dry out. Two sides? Top and one side that connects to the grating? Allow it to lean against the head platform?
Do you need to cut out an arc for the door or not? Because installing and removing it all the time would be a PITA.
OK, everybody think on this and let's come up with something useful and simple for NAM.