Cữ đánh dấu trên thước (Ruler Marking Gauge)

I designed this to fit on a ruler 2" wide and 3mm thick (yes I mix units.  It's gonna be OK.)  I set up the 3mf with a layer change operation that highlights the crosshairs around the marking hole.  Use a contrasting color to enhance the visibility when using a laser level or somesuch.

Intended to be used in pairs, but there's no reason you couldn't print a bunch and make a story stick.

This works on a ruler I got at the home center, and also fits fine on a drywall square.