The existing 626 bearing based idler may not fit the timing belt pulley on the Printruder. This is a redesign based on the narrower 688 ball bearing (4mm depth) that will fit the pulley.

Thanks to Zaggo for the awesome Printruder II

Substitute the 688 ball bearing for the 626, use a M8x20 bolt.

I have also included a sketchup template of the idler block in case you want to try other sized ball bearings. I have also added a thin 1mm wall to the bearing end of the idler so that it prints better, and provides additional reinforcement.