In 2 pieces to maximise strenght, doesn't require any hardware. Has markings by every 1mm, 5 and 10mm are highlighted!

Installation: 

Push the square into the bottom piece as far as it goes and then use a hammer to tap it in all the way.  Very tight fit because we want everything to be square!

If you want markings and lines to be different color on the upper piece, then add pause at layer #41 and swap out the filament to a different color.

Print settings: 

I've oriented the files as they are supposed to be placed on the print bed.

Layer height: 0.12 - 0.20

Walls: 3 or more

Infill: 15% or more.

Supports: No

Brim: Mouse Ears.