I really like AdanA's EZ-Snap Razor Blade Handle, but since I have pets in the house, I realized it wasn't a good idea to leave them just lying around. I came up with this cover after spending a few hours fiddling around in SolidWorks.
It snaps onto the handle just like the handle snaps onto the blade: you insert the front into the groove then snap the handle down into place. When inserted, the bottom of the cover sits flush with the bottom of the handle, which makes it really easy to mount onto flat surfaces with just a little double-sided tape. I might make a version with holes or something for easier mounting.
I've also included the original SolidWorks file in case anyone wants to fiddle around with it a little.
The print is pretty easy, since the entire bottom sits on the bed and there aren't any bridges or overhangs over 45 degrees. The only thing is that I tend to print the handles at a 102.5% scale to allow the blades to snap in more easily, so the cover fits those. If you have normal-sized handles, scaling to 97.6% or so should do the trick.
For mounting, I just have it taped to the side of my Printrbot for easy access, but you can put it anywhere you want.