A spool keeper for my one piece spool holder: http://www.thingiverse.com/thing:2036761
And my Duplicator 6 spool holder: http://www.thingiverse.com/thing:2002418

It allows you to give the spool more drag to keep a pesky spool from tangling up. The holder contains a clip that you flex like a clothespin when installing on the spool holder.

Print in a rigid but flexible material like ABS, PETG, or possibly a modified PLA+. If printed in standard PLA the clip will most likely break (and possibly explode across room) when attempting to flex clip.