This is designed to be used with this digital depth gauge: Check around, you might find a cheaper one on ebay, but it'll probably ship from china.

You can use two zip ties to mount the gauge. I designed this to mount where the extruder fan normally sits. The depth gauge should be lined up with the extruder, I expect this could be kept on the printer if you use the Cobra extruder fan, or some other rear mount fan.

I haven't had the chance to print this out. It may need adjustments and is most probably over designed. Let me know if you have any suggestions for corrections.