When I had more keys on my keyring I used something like this https://www.amazon.com/Lucky-Line-Keychain-Nickel-Plated-707... (not this particular one, which is just the top result that looked similar). It worked pretty well for a single key, but if you want to use more than one it adds a lot of extra bulk so your keychain gets even bigger.

I also tried something like this https://www.amazon.com/Spider-Accessory-Split-Rings-Pack/dp/.... It worked pretty well for a few weeks, but then the central piece loosened up and the keys started falling off in my pocket; not recommended unless you can find one that's really sturdy.