I disliked the wobbly design of the Ultimaker Original spool holder and created a fitting spool bar inclusive lock for dual extrusion to keep filament spools working smooth. Just remove the small parts of the spool holder and replace them with the printed bar. Put the two spools on and lock the outer one to avoid slipping off the bar. The friction caused by rotation should be very low when extruders pull the filament. Hint: Just cutt off a piece of the edge in the middle of the bar (left and right) if the first spool does not fit into place.