Summary

This trusquin is simple et efficient and easy to use
https://www.thingiverse.com/thing:4792970

however, I got problem to print it with and without supports. (the ruler stuck inside)
In addition, I wanted to use different rulers format.

So I made a remix where I cut the trusquin in 2 parts, and in addition, it's easy to print a new lower part to fit any ruler size.
I used inserts, so the object can be quickly disassembled and it's stronger.
You can use simple screw if you change the lower hole size to 3 millimeters.

I joined the freecad files to permit to modify as you need