Prusi I3 Mk2 Revised Spool Holder

Another option for a fool proof spool holder, uses a 3/8 in dowel rod as the holder, inserted into the spool knobs and lays on top of the notched supports allowing for quick removal of a spool.
Update: I added a printable spool rod in the event you do not have or want to use a 3/8 in dowel rod.