1) Print 3x spoolArms and 1x spoolHub.
2) Cut 6mm aluminium tube to suitable lengths - I used 180mm lengths to give an outer reel diameter < 300mm
3) Drill out the arm holes to 6mm
4) Insert aluminium arm tubes
5) Slide on plastic arms
6) Cut 12mm aluminium tube to suitable length for the axle - I used a leftover piece and suspended it above my printer using wire.
7) Load filament - easiest way is to slide one of the plastic arms against the hub, then loop the filament over other two arms and finally slide the last arm back out to hold the filament securely
8) Slide spool onto axle

If it's a small reel of filament, then twist the arms sideways to hold the filament securely.