I tend to daydream while swimming laps and forget my lap count. I made this to help me remember my count. It's easy to increment it with your thumb. The number visible just above the thumb is the count. It repeats after 5, so you still need to remember how many times it's gone around.
It has a one-inch inside diameter. When you slice it, scale it to fit your index finger. For example, if your index finger is 7/8 inch in diameter, scale it to 87.5%. If it is hard to rotate it with your thumb, make it a little bigger. If it slides off, make it a little smaller.