hope someone can slot port

Hi everyone, I usually just build fiberglass ensclosures but im just building a regular square box for my wifes trailblazer. This is the outside deminsions i was going to make it. Its for one single 12"

16.75 wide / 19.75 high / 19.75 long

looks like thats about 2.7 cu feet the with the speaker displacment of 0.18  looks like 2.939 without speaker displacement.  manual calls for anywhere from 2.3 cu feet to 3.7 for ultimate spl. so i was thinking the 2.7 should be fine. could be alittle longer if needed.

i have no clue how to calulate the square port ? does it change the inside box volume? if some one could give me some ideas it would be great. manular said for the 2.3 cu feet box tune to 38 hz. Thanks so much i appericate it.