608 Idler Pulley to fit onto a standard 608 zz bearing with 25mm centre groove diameter and 30mm outside diameter.
A standard 608zz bearing will fit onto the stand which can then be bolten together with a M6 nut.
More info at my site: http://bit.ly/1bbeXHQ
Update: I have rotated the model a bit so it should now need fewer supports when printing.
Update 2: This pulley should align perfectly with my stepper motor threaded drive pulley at the bottom for nice parallel lines. Get it at http://www.thingiverse.com/thing:159208
Update 3: I've added https://www.thingiverse.com/thing:2609737 to be used with this model as a guide to keep what ever you pass over the pulley centered. It is designed to fit on the side of the pulley and uses the same 6mm retaining shaft as the original Pulley and stand. Add 5 mm to the shaft length to accommodate the guide or 10 mm if you want both an entry and exit guide.