Scott,
I ran into the same problem about a year ago. I had no idea my square wasn't! I used a technique posted here and found out it was waaaay out of square! I bent it into a accordion and looked for a new one. After reading a bunch I found out that there are about three categories. $10-$15 (Borg stuff), $50-$80 (Lee Valley stuff) and $200-$300 (sterrett stuff). I opted for the middle road. It isn't perfect but usable; maybe a pencil mark off in 24".