This is a ruler stop which allows the easy accurate marking during metal or wood construction works. the M8x25 part is the one designed by Devious (Metric bolts and nuts fit steel Metric bolts and nuts (V2) by Devious | Download free STL model | Printables.com).

Since tight tolerances are required (I used 0.2 mm tolerance between ruler and stop opening) I'm including also the freeCAD file to adapt accordingly. My ruler had a cross section of 26.30x0.90 mmxmm.

Prusament PETG was used successfully.