The 15 1/2" square up ruler might be an option so if you go that way, go larger and get the 18 1/2" square up ruler - it's great for squaring up the corners of a quilt before putting the binding on.

That said, when I don't have a larger enough ruler, I use the two ruler method. I have a nice 3" X 18" ruler that I can place next to the 12 1/2" ruler. Takes more time but if you just don't want another ruler the two ruler method works.