For quite a while i was carrying around a 3D printed bottle opener on my keyring (http://www.thingiverse.com/thing:9500) which was rather bulky especially when considering i only had one key on it!
I remeber finding this http://www.thingiverse.com/thing:11753 and liking how the Author wrote:
"The conventional key ring is not very conducive to pockets. The plane of your keys is perpendicular to the plane of the ring, forming a shape that cannot lay flat. There is always some edge poking into your leg."
That caused me to redesign my keys and this is the result.
Note: this is designed for a Australian 5c piece so it may not work with other coins

Print both halves, add coin and key!