no matter how your port is shaped, the height and width, or diameter, need to remain constant.
in that pic what would the port length be? 32?? never seen this done....don't think i would recommend it...
i think i see where your going with this though. if the port is 6" wide for the 21" part, to be fare this part is where two ports merge into one. if the 11" each way was only 3" wide, it would seem to make sence. however, i would still divide the port in 2 down the middle to keep the 3" constant.