Summary

A simple tool to make your ruler into a caliper, just clips on. To modify to fit your ruler dimensions either go to:
https://cad.onshape.com/documents/96b4f8569a5b3a69790c968d/w/1e95fdebd6d10c98f5c6081e/e/9540a710afc641c4d3843ec4.

Or use the solid works project file included. To ensure good grip of the static part use 0.1mm clearance and 0.25mm clearance for the the slider.