I could not find a dial indicator mount that was easily printable and would accurately mount on my extruder (not sure if they changed the design recently) so I designed my own.

It uses magnets to keep the mount stable on the extruder, but locates itself by hugging onto 3 bolts on the outside of the extruder.

For reference, my SD3 bed had 0.31mm variance between the high and low point on the bed.

Magnets used. (6.2mm OD)
http://www.amazon.com/dp/B001KV9RAW

Dial indicator used
http://www.amazon.com/dp/B006YD7G36

After printing, drill the screw hugger holes with a 5.5mm drill bit. It will be a perfect and slightly tight fit, which is what you want. you might also need to slightly sand the dial indicator mount, but again, you want a very tight fit to keep it dimensionally stable. After printing, I can take the mount off the extruder, put it back on and be within a few hundredths of a mm. This makes it easy since you can find the nozzle contact point, zero the indicator and then you are just looking for the zero indication on the bed everywhere.

You should probably print this with fairly thick walls to keep it rigid and use supports to keep the overhangs from sagging and changing the dimensions.