I found this great handle http://www.thingiverse.com/thing:129760 for hex wrenches. However it was for only one size. I've create a customizable version so you can create your own for different size keys.
The best approach is to use the customizer, although you can download one of the STL files if it matches the size of your key.
To use the customizer, measure the width of the key, when rotated so it's the widest (as shown in the photo with the digital calipers). Enter this number as the diameter for the hole.
By default, the slot is increased in size by 0.15mm to compensate for differences in printing. You can adjust this to make it looser or tighter.
You can also change the diameter of the handle. I prefer the 25mm size that is the default.