Cutter Ruler

NOTE:
Meanwhile there are improved versions available at https://www.thingiverse.com/thing:3099863 and https://www.thingiverse.com/thing:3103154.

This adjustable ruler supports dragging a cutter knife along a border in a defined distance.

The ruler is printed as two parts to be glued together with CA glue to save support material.
Glue a M3x12 threaded rod (or a screw without head) into the slider and insert a M3 nut into the fixing knob. You will need some pressure or a pre-heated nut for insertion, but then it will be reliably fixed.