Get a micrometer head. I found a good deal on a 0-13mm one on the favorite auction site, but 0-25 should work too.
Obtain a length of 6-32, M3.5, or similar threaded rod. You'll need somewhere around 8 to 10cm, depending on your micrometer head.
A locknut, or pair of nuts, to fit
A compression spring that fits around the thread, extended length about 40mm. Not critical and not very strong , it's to hold the althread against the head - I got one off the display at my local hardware store.
Measure the diameter of the collar on your mic and adjust the OpenSCAD file if necessary. Print the holder - I used blue repraper PLA, 0.2mm layers, 100% infill every 2 layers. An additional part is included to go on the top end of the allthread and over the shaft on the head - tap the small hole to fit your allthread. I never used it.
Remove the original 6-32 Z adjuster screw and make sure the holder fits - the bottom tab should rest on the original plywood piece, the top should hook over the left (X0) side of the bridge. Recalibrate and reprint, or file to fit, as necessary. The small holes should line up with the original screw hole, too.
Clean up the holes for the micrometer head and allthread, if necessary. There's a horizontal hole in the top of the bracket (points to the right) for a M3 SHCS. Mine was close enough to self tap, but run a tap through it if you need to. Install the micrometer head, point the scale towards the hook on the top bracket, and gently lock it in place with the SHCS.
Put the allthread through the bottom hole, put the spring between the two tabs and compress it enough to get the locknut on, screw on the locknut (might require an extra hand or some strong language). Adjust the micrometer to zero, then screw the allthread up (or the nut down) until the spring is compressed a few mm after the allthread touches the micrometer.
Adjust the micrometer to about a third to half of it's travel, them measure how much screw thread is below the plywood tab on your current adjuster and add 6mm (the thickness of the plywood). Cut off your allthread so this much is below the bottom tab. If you're confident in your measurement use half the mic range first, if you think you're a bit long or short go up or down as appropriate. You can always cut another piece... File the end flat if necessary.
Prop your printerbot's bridge up, run up the Z, or remove, so you can get to the underside of the Z adjuster tab. Enlarge the hole so your allthread will just slide freely- you did check it was in the right position in the step above, didn't you? I used several drill bits in increasing sizes, turned by hand, and a bit of small round filing. If the bridge was removed, Drill, baby, Drill. If you change your mind and go back to a screw adjuster just epoxy a nut on the tab.
Put the gadget in place, sliding the althread through the original hole (printing off? file some more). Recheck that the hook goes over the top left of the bridge, and the bottom tab sits on the original adjuster. File if needed.
Cable tie it in place, one small tie to hold the bottom tab to the original adjuster, one on the side as high as it'll go to hold it snug against the side.
Turn the knob to rehome. Enjoy!