idea for a sub box
 

So I am not to fond of the sub boxes on the market for the 370z. So I would like to make a custom box to go right behind the metal bar that is behind the seats, but have it be flush with the trunk floor. My idea is to have two 6.5 inch subs and then two 6.5 inch component speakers on each side of the subs. So essentially the box will have four 6.5 inch holes in a line parallel with the metal bar on the trunk side. I know there is nothing under that part of the trunk, it is just a matter of getting the box made. However, I do not know how the two 6.5 inch speakers will sound if they are pointing up like a subwoofer. Any ideas?

You could angle them, like bigaudiofanatic's pods. I have been told that the reason speakers and subs are almost always separated is it is difficult to have them not interfere with each other.

For a set of rear fill speakers, that should work just fine for you. just be sure to keep the subs in their own separate enclosure inside the box.