I hate split rings. I like small steel repair links from the hardware store. I use the ones with a threaded hex nut that closes the link. They are tough as nails. The smallest size fits most keys as they are but there are a few keys I've had to file just a bit on the hole to make it a little bigger.
I started using them when I was a scenic carpenter. Crawling around on the floor can tear up most keyrings worn on belts. The repair links are indestructable and once the keys are on it and the nut is tightened a bit with a wrench -- that's it. Nothing you'll survive will ever open one up accidentally.