Worm on the string! For your thing! Not only does Dilbert make a good c-ring, but he also makes a nice bracelet if you're brave enough. And you know what they say, if you love it put a Dilbert on it.
This is the thicc version for folks who enjoy a tighter fit.
Slim size has a rough size of 3" in diameter outer measurement and a 1.5" inner diameter.