This turned into more of a project that I had planned, but I am very happy with the result. The 3mf files for the squares have filament change stops so that the graduations and numerals are contrasted from the surface. If you want a single color just re-insert the same filament. The extension to 30 cm is completely optional, no need to print it if 24 cm will do you. If you want it clean out the slots on the end of the square and extension with an xacto knife, insert the biscuits and super glue it together if using PLA. Super glue the non rounded side of the clip in the slots on the square and you are ready to go.

Added a inch and centimeter Tsquare with FreeCAD file.