I modified the original design (http://www.thingiverse.com/thing:250957) to hold the dial indicator by gentle pressure on the outer tube. This means that the sensor is free move, but the body is tightly held by the mount, thus providing reproducible measurements.

I also modified the "clamp" that sits on M2 rail to have fewer friction points with the rail. This allows for tight fit but at the same time the ability to slide the dial along the edge of the rail.

I printed this in PLA standing up-right (so that the central bore prints clearly). I used infill support to get the upper edge of the "clamp" part to print precisely.