Clarification: I am using a geared stepper with an 8mm output shaft. My hobbed gear is from Makergear and is 13mm in diameter with a 11mm diameter for the groove (Trinity Labs website has the same size). The M4 screws are 45mm long.
Updated 12/28/2013 - added some support for the pushfit. I had an issue with the first one's threads getting pushed out. Also, instead of cutting threads I use a hotair station to warm the plastic and then screwed the pushfit in forming the threads instead.
I am using it with a 5.2:1 geared stepper. I modified the scad file to remove some un-needed bulk, extend the mounting feet to work on my 15mm extrusion, and add some roundness to the part. I like the idler on this model, it holds the 608 bearing nicely.