I gathered up my existing parts and with a little help of cad and cardboard, I threw together some prototypes to get a feel for the size.  I ran into possibly extending the width another 25mm or so to get some usb panel mounts on the side of the lower section. 

With the size being around 460 x 200 x 70 [18" x 7.5" x 2.75"], another inch or so is worth getting another port.