Nice design! I really like the printable tensioning screw.
However, I had some trouble with the nut attachments. They seem very hard to print and very brittle, so they easily break when trying to tension the belt properly. I'm going to keep using http://www.thingiverse.com/thing:1492411 for now, and maybe I'll try to print this again or make a remix without the 4 extra screws when I have time.