http://www.thingiverse.com/thing:69029

The Z holder has been updated, so I only focused on X and Y.  Open up the idler arm and slide the mount over the top of the extruder motor.  If you have an extruder with a zip tie on it, this won't work.  I'll need to come up with a workaround for it.   The plunger on my gauge just happens to hit the edge of the frame.  It could shoot over the top, but it would be easy enough to make a clip to slide on to the frame to give the plunger something to push against.

Test for backlash by pushing the gauge against the frame a few mm, then reset to 0.  Run 10mm toward the frame, and 10mm back.  The remaining distance on the gauge is the amount of backlash.  Mine was .4mm to start with.  I loosened the Y motor screws, pulled the motor down a little, and tightened the screws back up while I held it.  After that it read .18 of backlash.  I tightened it up a little more, and then got a reading of .08 which is good enough.  Each time I tightened the Y motor belt I didn't see it move, I just pulled until I felt it give a little.  It's possible to pull it too tight, so run it back and forth the length of the axis to make sure it isn't going to bind.

The slots are a tight fit for the tab on my gauge.  I would be interested in hearing the measurements of other gauges so I can adjust the size of the slots to accomodate a wider range of models, using a set screw to hold the smaller ones tight.