78' Trans am Box Questions

To start off this would be my first box ever, and from making one out of cardboard first, this freaking this is going to be huge.......

this is what I have 4'' 12's audiobahn immortal series

this is what I came up with there not final or anything like that, but if someone could tell me im on the right track, and what not to do while building this....

I've read all the things on this site pertaining to box building, well alot of it....I went to use the Volume Calculator - Rectangular/Square Enclosures

and this is what I came up with for the bottom to speakers

this is what I got for the top subs

This is all going in the back seat, or where it use to be, But as far as I know the back of this box will be all one sheet of wood, or whatever I use, but I do not know how to calulate....how to you say this...making it bigger so you dont offset the cubic feet of the box,,,, or how about adding ports? I've seen some that are built with the box and others that arethe plactic type, which is better to use.......

well I do appricate it, and thank you

Kansas