Simple ruler holder for up to seven rulers.  
4 option total

1 - Original design for machinist rulers up to 1.5mm thick (This is the primarily in the photos).  

2 - A remix of the original to allow for thicker rulers up to 3.5 mm thick

3 - A version 2 of the original, allows for 1.7 mm thick ruler and after several months of this at the back of my desk draw, I added notch on the top to ease the re-entry of the rulers. 

4 - A version 2 of the thicker ruler set