I couldn't find a Printrbot spool holder that was simple enough to fit on top of the printer so I made this one. This fits the 6x6x6 printer. It angles a 1kg spool at 45 degrees in order to keep the machine balanced and gives the the z-motor clearance.

Works fine at 0.4mm resolution. I added a 10mm brim to help stabilize it as the arm is built.