I found it a bit challenging to get the Y-axis belt tension just right by using the belt clamps, so I made this Y-belt tensioner in SketchUp. It allows for more accurate tensioning of the belt using a M3 bolt and captive nut. The design is based on the belt tensioning system used in GregFrost's LM8UU X-Carriage (http://www.thingiverse.com/thing:9869).
The holes have the same spacing as the original belt clamps, so you should be able to simply replace the existing clamps without drilling new holes.
The tensioning bolt can be reached with an allen key even with the build plate mounted on top.
Sorry for the SketchUp design. I haven't taken the plunge into parametric modelling yet...