Nylock Nut Knob

I've been using the [Prusa Mendel Filament Spool Holder](http://www.thingiverse.com/thing:14999) for a couple of years now (I should; I designed it!), but I've been using a box wrench whenever I needed to change out filament reels. I belatedly had the idea of making this knob (which press-fits onto a 5/16" nylock nut) to make it possible to do filament changes using just bare hands.

It's a good refinement, and makes me a bit more willing to, e.g., change filament to get a different color.