My keychain consisted of a large key fob and two rarely-used housekeys. I wanted a way to carry the housekeys in my wallet instead of having them clang around against the key fob in my pocket. This will hold two Schlage keys in a credit-card sized device, only .4 mm thicker than the keys themselves. It will work with the standard sized Schlage keys as well as the slightly longer keys used in their anti-bump locks.

It may work with other brand keys as well, I haven't tried it. My keys press-fit in nicely, but if your key is loose you can always use a little double-sided tape to hold it.