What's the problem with a 0-10 mm ruler? I would clip that onto a keychain and use it to check gaps and alignments. In fact, I may steal this design and start 3D printing them for profit.
And in the spirit of the Uncomfortable Ruler, this has the same frustrating design flaw: the scale doesn't start in the corner so you can't butt it up against something and use it like a depth gauge!
OH, its already been done!
https://www.printables.com/model/689367-1-cm-ruler-keychain