I was talking to Gernot Wagner at a guitar festival a few years ago - he showed me his version of the Hacklinger, which his engineer friend made. It's a beautiful piece of engineering, and probably (I wasn't able to test it) more accurate than the Hacklinger. Gernot asked if I'd like to know how to make one for a few Euros. Guess what I said...
It's a similar principle to the Hacklinger, but uses three magnets, rather than 2 magnets and a spring. This makes the scale linear I think (certainly closer to linear than the Hacklinger). Calibration is quite simple, and so far seems pretty stable. Accuracy is probably about +/-0.1mm, maybe a little worse for a single reading, a bit better with multiple readings. It uses a shallow wedge (wood) with the calibrated scale marked on it to gradually seperate the top magnet from the middle one. You can probably work out the rest for yourselves - I'm just trying to remember if I promised Gernot not to disclose the design!
James Lister, luthier, Sheffield UK