Here's an "always-on" style touch probe I made to probe off boards for heightmapping, so that I could skip planing the board entirely and just v-carve directly onto the board without the cuts coming out all wonky because of any slight warping or bowing. I designed the probe up in DesignSpark Mechanical to be made out of a stack of 0.25" thick acrylic that's bolted together. The shank is just a cotter pin, which actually ended up being slightly smaller than 0.25" and just barely gets held by my 1/4" collet.
