I thought I would be able to push one of these (http://www.thingiverse.com/thing:9864) onto the smooth rod... I was wrong. It snapped. So I redesigned it for the same purpose in a retrofittable, push on style. Press fit a 608ZZ bearing into the holder ringer, push onto the smooth rod and maneuver it into position. Then tighten across the end (larger) through-hole with a M3 screw. The other hole (closer to the throat) is intended for my fine adjustment end-stop mount (http://www.thingiverse.com/thing:158872).