I say bite the bullet and buy the 12.5" square up ruler, too. I tried all (except going to the hardware store for plexiglass) and NONE worked for me. Even though I tried to square up my own, I always ended up with a skew. Yeah...12.5" this way and 12.5" that way.....but they all leaned. I tried with different rulers, different mats...... ARRRRGH What I found with the 12.5" SUR I just bought......it has all kinds of markings on it that will help with ANY quilt block size smaller than 12.5".......