A new Gorilla Grip shower mat defeated the spring-loaded clip its predecessor dangled on between showers, so I applied a contour gauge to the shower stall, copied the shapes to paper, scanned them, then fit some curves:

Combine the two to get a model of the shelf, subtract it from a rectangle, and produce a cardboard test piece:

Put a hook on the bottom, a small bump on the top, and round the corners:

Function-test a cardboard pair:

They clear the hump along the edge and fit snugly across the front and sloping underside, so make an MVP from 6 mm acrylic:

Laser-cut acrylic may be too brittle for the job, but it gets the mat off the floor for drying and we’ll see how this works before doing anything more complex.
Spam comments get trashed, so don’t bother. Comment moderation may cause a delay.