Spring-loaded sliding mount for a good old mechanical micro switch. Has about 6mm of travel (1/4") so get it close then fine tune the deck height. This works really well and is a nice example of close tolerance sliding parts printed in PLA. Found these cheap and nasty micro switches on fleabay for a few cents each. http://www.ebay.com/itm/261071015810

Print it out - one file does the three parts. 3 perimeters, 3 layers, 0.35 nozzle, 0.25mm layer height, 25% infill at 90 degrees fill angle. Fit an M3 nut to the hole in the thumbwheel - would be a press fit but if it resists just push it in with a hot soldering iron. Run a long M3 screw through the thumb wheel and tighten it against the nut. Find a suitable spring - I used on from a ball point pen - shorten it to fit in under compression, about 25mm or 1". Fit another M3 nut in the top of the fixed part then assemble it like the photo. Fit it to the smooth rod below the z axis below the lowest point of travel.