A conical adapter for the i3 MK2's filament arms. This lets MK2 users mount spools with asymmetric sides, or spools with odd inner core diameters (compatible with spools with a ~> 38mm inner core diameter).
3D printer, filament.
6x 608 skateboard bearings (3 for each holder). Bones Reds have been tested with this adapter and work reasonably well.
When mounting a spool, the arms might snag slightly on the sides if the arms are spaced too tightly. Post installation, give it a spin to test and adjust as needed with the spool mounted.