spool rollers

I used some crappy 4x13x6mm V-groove ball bearings I had left over from my 3D printer; you can easily find them under "V-groove bearings" on Aliexpress for pretty cheap. The U-groove version should work even better. Careful, the (much better quality) extruder idler bearings won't work since the groove is too narrow for the rim of most filement spools.