I created this 5x4 gridfinity box to hold my Starrett Combination Square and Protractor heads.  I put the two heads together rather than individual boxes as this let me fit both in a 5x4 box instead of needing a 5x3 and 4x3 box.  I also created a 1x1 rack to hold the straight edge for the combination square along with other straight edges/rulers.

You can also use my straight edge holder to store the blades of your combination square.