in the companionway steps 50 years ago. They're about 1/4" deep, the ends rounded by the router bit. The slots contain 1/2" strips of raw teak(with easily rounded ends) which stand proud about a 1/4". The slots contain the strips securely, and a few small brads keep them in place. They seem to last forever, so far but are easily replaced when they wear out. Sort of sacrificial as the stairs themselves take little wear.

That's one idea anyway, there are no doubt many simpler ones as well. It is one area you want a sure grip for bare feet or shoes.