In my opinion top mounted Printrbot spools make the machine top heavy and are a bad idea due the small base surface area.
This is my version of machine a DUAL back mounted spool holder based on http://www.thingiverse.com/thing:846520 design. I wanted to reduce the width to
60mm for common
1kg spools yet still allow for wider spools. To do so I eliminated the end cap and to prevent spools from falling off I slanted the arm
2% angle - cross your fingers I'm printing right now - DONE, looks like it will work and after first few prints I have done further changes:
- removed the starting cap too, the spool stops on the bottom triangle like support
- moved the screw hold out
0.5mm to make the spool hug the metal edge better - keep me posted on this change as I choose not to re-print the original left spool design.
- added a new version that can sit on the right side of the printrbot for holding a second spool