first off, the height of the port should be 15.5" (I had the cell referenced, you must have deleted it).
and unless you add a corner to the port it won't be that long.
without adding a bend to the port (one straight wall, stopping 4" short of the back wall), i get:
4.14 ft^3 @ 36.33 Hz
you need to learn how to use WinISD //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif