Quick displacement question

[shizzzon 461]

a box's internal dimensions -

10.5" high

35.5" wide

20" deep

If i put a port in it width-wise, slot port along the wall, 10.5" high x 4" wide, and run it across and leave a 4" gap from the wall resulting in port length being 31.5" long + 0.75" for wood thickness exiting box...

When i calculate port displacement, do i find the cubic volume of 10.5" x 4.75"(plus thickness of wood) x 31.5"

or

10.5" x 4.75" by 35.5" ?

Do i take into account the effective area around the port too? because if i do then i would choose the 2nd option above as part of the displacement.