My choice fell some time ago on some rubber pieces taken from an old bike handle. They worked, but they were ugly and difficult to glue, maybe you don't believe me but they always came off, whichever was the glue!
So I thought to something more nice and tough, and the solution came in a Sugru
packet! I choose the yellow mini pack because it will stand out well on the black plastic (and the general black colour of my bike and gears).