This is versatile spool holder for Prusa I3 MK2/2S/3 frame and not only. I am using the Area51's clever ClipOn design. There is also a version for aluminum extrusion frames . Unfortunately I do not have aluminum extrusions to try it , so if someone makes it, please give feedback.
You can use smooth rods or threaded rods of 8mm,10mm,12mm diameter (one version for each diameter). With smooths rods you can use something like alexrosa's 3D Print Spool Holder Filament Support or boksbox's hub adapter. If you prefer a hub with bearings you can print a design like butchja's Universal Filament Spool Holder and mount it on threaded rod. You can use it as single filament holder or double filament holder (see photos and renders).
I also included a clip (one for each diameter) for smooth rods.