A printable bearing for http://www.thingiverse.com/thing:24526#
A 608 bearing seemed overkill when the filament really only uses the guide to load the filament from a fixed point, so I made a lookalike bearing to keep the cost down. Functional, but not ideal.
SketchUp file included.
Fit it in the guide
See what needs modified
Remove and fix with a knife
Finish with instructions from http://www.thingiverse.com/thing:24526#
I used 3 perimeters and 50% infill for this small part
I only needed to round the edges on the center of the bearing to allow it to spin a little easier.