simple project to use my digital caliper as a depth micrometer. One day I'll get a proper metric depth micrometer, but until then this should do fine.
Made from a perfectly sized piece of 13x100x25mm alu scrap. Drilled two holes in the side (where the rounded inside corners are) either side of where I wanted the slot to be. Drilled and tapped 2 matching holes in a piece of scrap and then screwed them together. This allowed me to hold my work in the vice but machine all 4 sides AND the slot in one set up, so that the slot was perpendicular to the base.
Cut the faces with a 1/4in 2 flute endmill, then the slot to ~7mm deep and ~9mm wide. Followed up with a woodruff key cutter - first the thinnest one I have, then one closer to size (~3mm) - and did a few passes to get a tight sliding fit with the caliper.
Then cut the chunks out with a hacksaw, cleaned up the sides and then angled both tops 3deg to clean them up.
Drilled and tapped for 4-40, cut down a thumb screw I salvaged from somewhere and tarted up the edges with a file. Done!