You mean the front & back should be 13.25"h x 20"w.
I always sucked at Math, but let me see if I got this correct. The 2 sides should be 12"w x 14"h, the top & bottom should be 20"w x 12"h, and the front & back should be 20"w x 13.25"h. Making the box flush mounted, making the deminsions 20"w x 14"h x 12"d.
Am I right?