I used redwood 2x4's stacked. 2 layers = 7 1/2 actual inches. The box is only 3ft. x 3 ft. because it was used by children (shorter arms).

It's the great mystery of measurement in the world. 32" TVs are only 31.5", and 2x4s are shorter than they are too! But it's easier to say 32" and 2x4.

In the grand scheme of filling soil and using SFG growing methods, 11.5 or 12.5" will be OK, meaning that your wood or string can just be off by a bit and the plants won't mind.