Summary

A square for marking three sides of a board in one setup. Sized for marking 2x4s, fits over boards 1.5"(38mm) and narrower.

For best results I'd suggest using a 0.2mm layer height, at least for the dovetail portions, I used variable layer height and did the rest of the parts in 0.3mm height. 

No supports are required, the parts need to be printed in this orientation: