When I realized that I needed more tension on my belts I found that standart Y-Motor mount is not strong enough (or not so well designed) to keep its place. It twists and driver gear stop beeing completely aligned. Fortunately i found some old bearings that just fit 5mm inner diameter and I designed a kind of mirror for the standart mount, so motor driveshaft end stay on the bearing and i can fit the belt as much as i need without any issue.
Design is for a 16x5x5 bearing and lower screw is replaced by a 15mm one to join with original mount.
If you wonder which pulley gear design I use in addition to my design: http://www.thingiverse.com/thing:740311
Please feel free to update or upgrade, you are welcome!!!
Use a wood piece or some other soft material to press bearing on its place.
Replace original screw (mine was only 8mm) by a longer one but not too much, a 15mm screw will fit great.
Use some nuts and spacers to place both pieces (original and couple) as parallel as possible.