6 10" sub box help

[tayodell41]

I am planning on running 6 American Bass XDs in the trunk of my 2001 Toyota Camry. My max dimensions are 40" wide, 25" deep, and 15" tall (yes that leaves plenty of room for the subs being mounted sub up). My main reason for this post is that I would like to do a square port that is partly external of the box, so that it can fire directly into my car where my back seats fold down. I haven't worked with square ports before and was wondering what the size of this port would have to be. Any help would be appreciated on how to figure this out. Thank you.