Channeled Marble Towel Hook

My wife asked me if I could design and 3D print a “hook” to hang our shower mat/towel for drying. I wanted something that wasn't going to protrude out too far from the wall or that something could be caught on. After searching around, I discovered the idea of an internal ball hook and came up with this design.

This design utilizes a 15.8mm diameter glass marble, which I assume is a common size since I had several of them laying around.

I made this particular towel holder with 2.5mm in clearance which is the thickness of the border on my shower mat. This gap should be enough to work properly with a standard bath or hand towel, as well.