Since i didn't really like any of the existing belt tensioner solutions i came up with my own. I wanted something that was not too big and also looked decent and was easy to operate. So i think i have met all three requirements. For this thing you also need a thumb screw so i took it from another thing; http://www.thingiverse.com/thing:16428 but feel free to use anything you like. The nut and bolt are M3 (metric) and the bolt is 20 mm long. BTW: for the belt to clamp inside the little retainer i used a piece of filament cut to the right size.