I printed the filament drive found here: http://www.thingiverse.com/thing:7113, but I found myself lacking an M6 bolt + nut.
Because the bearing just needs to be held in there, but not tightened down, I decided to design a pin to do the job. This pin is 19mm long, with 3mm of base, and 3mm for the flange at the end. The base will sit ~1mm from the back of the filament drive when it is engaged, so the pin is incapable of coming out while in use.
Print, if your bot is reasonably well calibrated to size the pin should need minimal sanding (I had to sand the flanges down a tad).