Umm Yeah, But you said you were building it on the outside. It's not INSIDE. No displacment.
If you are building a port that goes in the box and out only factor displacment on the portion in the box.
It's common sense on this one //content.invisioncic.com/y282845/emoticons/eyebrow.gif.fe2c18d8720fe8c7eaed347b21ea05a5.gif . Port in box = factor displacment, Port out of box = no displacment, port both inside and out = only factor displacement on the portion taking up space in the box.
If you still dont get it, give me your box dimmensions and a drawing of your plan. Also speaker displacment, And I will tell you the length.