NOTE (Oct 2016): for an alternative design that uses bearings to minimize the friction please see here http://www.thingiverse.com/thing:1728291 . This simpler design here should work just fine though.
This customizable spool holder fits those Flashforge Creator Pro 3D printers models that come two horizontal slots at the back.
This design uses a new U shape hanger that makes the insertion and removal of the spool holder much easier. My printer is located about one spool width from a wall and replacing spools using the standard Z shape hangers is impossible.
With this design I just lift the spool together with the holder, pull it slightly out and it is released . Insertion is just as simple and the slops at the end of the hanger guides smoothly it into the slot.
To customize the dimension of this spool holder (e.g. the length of the rod) use the Customizer button on the right.
I printed mine using ABS, 20% infill and support material and sliced using Simplify3D.
IMPORTANT: make sure to print the model in the orientation it is shown in the blue model here, that is, on its side. This makes sure that the direction of the printing lines is along the rod and the hanger and provide maximum strength. This orientation requires material support but should be easy to remove.