I generally go with a loop of paracord when it comes to fitting slings to overly-narrow sling mounts on guns. Problem is that so many designs of airsoft guns are based on very, very old iterations of the real firearms and the slings used back in those days were much simpler affairs that just threaded a 1-2" canvas/webbing strap directly through the mounts on the weapon á la the famous L85 3-point.
The advantage with paracord vs a keyring, or any other metal or plastic solution, is that you'll get zero rattle or scratching from the interface device. Also greater flexibility, greater options for customisation (i.e. unlimited, it's just string after all) and IMO it just 'fits' better aesthetically.