I remember to have seen somewhere a solution like that, maybe on mendel-parts but I couldn't find the files or something similar...
Spool inner diam. : 52mm
Video of a free rolling : http://flic.kr/p/cucvJ3
Print the two parts
Assemble with M4x30 bolt and 624 bearings
Add to the frame with M4x20 bolt
A 650mm 2-4mm tubing will guide the filament.
Let the spool spin freely !