This uses http://www.thingiverse.com/thing:651469, or another 20mm round spinning thing with a 5mm bore. This was designed for my Tevo Tarantula, but can probably be used on most any 2040 extrusion as a idler. The design uses a m5 nut, 25mm m5, 2 m4x10 and t nuts, for the printed pulley you need two 626 bearings and 2 1mm spacers. The design has tight tolerances, but I was able to fit everything together easy enough. You can set the m5 with the 25mm if you need to.
Please post your makes, or pic of the rig with the parts if you use it. Thanks!
Use support on the nut hole, belt passage, and the larger side of the spacer. The belt passage can probably be printed with no support, but I would say the nut hole and spacer should be. It will help with accuracy and it's how I printed them.