For subs, as long as you don't have a perfect cube you won't have any cancellation issues. The size of the waves that subs produce are too big to have any problems.
As for wire gauge, in my experience anything larger than 16 gauge will be perfectly fine.
Also, I'm coming up with a different measurement than you are. I'm coming up with outer dimensions having to be 12 x 12 x 11.8 to have a .66 cubes box.