Summary

Use customizer to custom fit to any size rule.

This can measure length of 2 random points such as the back of picture frames, extension leads, towel rail etc.... and transfer the hole centers to wall for drilling.

It also can be used as a height gauge / comparison. Please note if using as a height gauge make sure your rule graduations start from the datum edge and not as a ruler does which start slightly in from the datum edge. (that's the difference between a "rule" and "ruler")

Another use is as a "L Square" drawing aid to draw parallel lines or reading data from tables.

The fit needs to be an interference fit with your rule.

I recommend printing this vertically as shown in the STL file
This is a quick print thing, I recommend you do a test print by stopping the print a couple of mm in height and test the slide fit with your rule then adjust the parameters in OpenSCAD or customizer and reprint and after a couple of attempts you should get a nice sliding fit which grips the rule nice and square with no wobble.

Hope you like the design as it will fit any size rule and fully customizable, if you need help message me.

Checkout my other designs / things
https://www.thingiverse.com/Ken_Applications/designs

Possible improvements... add an extra bit to turn it into a "T Square" and not a "L square"
Print a base to hold it vertical, like a height gauge so you can fix a digital caliper to it.