.... When it was hot and I was dripping with sweat, I found the twist grip shifter became increasingly difficult to use. ....
I think that Rohloff has had three generations of twist grips. First was a rounded triangular shape, second which is what I have was more rounded, and third has some sizeable wavy shaped bumps on it for better grip.
I added three beads of Shoe Goo on my rounded one to give me better grip. The rounded one also has a soft rubber that abrades easily, the Shoo Goo is harder material, does not abrade.
These photos are five years old, it is still working quite well.
Shoe Goo is sold in clear and black, the black is harder to find but I found some on Amazon and ordered it from there five years ago.