a box like the second one, w/o the bottom part, and with only one sub-woofer, centered

Just make a box just like that. as long as it's sealed it will be super easy to make.

i would if it were easy, i have the top piece cutout, but as far as figuring out the way to do the front/back piece its not so easy, the first box was easy, i had a flat top, then did about 1 inch away from the walls and bottem, with this box, the back, front, top are all at an angle, the sides and i'm hoping the bottom will be flat, i just can't figure out the way to get the front/back to work

measure each piece by piece if you have to, its pretty easy.

i have, thats the thing, it's almost like i have to box in the whole trunk, then make the box i want inside of that box

Or draw it on paper, measure the most you can possibly fit, and subtract the width of the wood and write down what each piece measures then cut.

that's the thing though, without atleast one flat surface to work with i can figure out accurate dimensions

What do you mean a flat surface?

more or less an edge to go off from to figure out if the lengths are correct

Take your own measurements lol