I found this key ring http://www.thingiverse.com/thing:245764 very useful for some time, but wanted to free my pockets. So I designed one specifically for my work keys and made a magnetic belt clip for it.
One side is sized for regular door keys and the other fits cabinets keys. Rectangular recess is meant to add a small ID and useful to check orientation when clipping it in.
It looks a bit chubby, but is small enough so its not in the way even when sitting. Make a nice schlack! sound when the magnets mate.
It uses 4 8x3 mm neodymium magnets and 3 screws and nuts (sorry I don't know exact sizes, they're printer leftovers)