I used a clevis pin in the stock pivot hole and attached key ring through the clevis' pin hole. You'll hardly notice (if at all) the flat end of the clevis pin in the palm of your hand. This works great for single point and hangs well off of my MS-3 sling.
For the ability to also have a 2 pointer, I attached a standard sling ring in the hole up front (I tried a key ring by itself, but it not only looked funky, it also was a nuisance to try to attach to), left behind by the removal of the original bipod stock. Again, with my Magpul sling (and I'm sure it'd work with many others too) I can switch between single and 2-point carry easily and quickly.
Hope this helps
W