This is a grip ring for a Grohe/Hansgrohe shower knob. I had the same problem as plumber1 - the standard knob provides little to no grip when wet and soapy. I've got an inspiration from this shower knob.

Given the printing area size of my 3D printer and the size of the original knob (50mm in diameter), I decided in favour of an inward grip element.

The round outdent on the inner circle should fit into the spot on the shower knob. Since these two parts are going to rub each other, I put a self adhesive insulating blue tape in-between so to prevent any scraps or damage to the original knob.

Now, I have one less problem and can enjoy my shower stress free :)

If your shower knob is different, please take the FreeCad file and adapt to your specific needs. Good luck and have an easy ride.