Hi, I have seen nitrile O rings used to help gripping pens.
Never done it myself but would seem to be quite easy when you think of it. Decide on the thickness of the ring, cut a small groove for the ring to sit just proud of the surface and away you go. You could fit about 5 or 6 rings at the end of the pen.
You can also get coloured rings, but don't know if in this country.
Google "rubber jump rings chain maille" will bring up the coloured rings in the UK. Just found it.