Well I wasn't saying that you have to downgrade. For instance I've got 2 15's in 2 separate boxes of about 1.5 cf each and my trunk is still functional. If you don't care about trunk space then go ahead and make the rectangular box but if you still want to use it then you're gonna have to account for the angle against the seats which will change the design of your box.
Also if you want to be able to get the subs in and out easily into would suggest doing two different boxes of the same size unless you wanna call a buddy over every time you wanna get them out.
