Unless I am completely missing something, he is only making a mock up right now, he will use the panel deminsions from his mock up to make a real box. Also that box is nowhere near 5 cubic feet. Isisde there is the wheel well, the back panel, and the gas filler tube, all which take up space, If he has 1.5cf or more I would be supprised. I know this because I was going to build a fiberglass box in the same spot. Next if he dosen't build a sealed enclosue or air tight enclosure a free air sub won't help anyway, he will get tons of air noise from any leaks and it will sound like arse. A free air sub still needs and enclosure, just a much biger one. If you run a free air (or infinite baffle) sub without sealing the trunk properly it will sound like arse too. He can make that enclosure sealed, but it will take some care to work around all the stuf on that back wall. Not at all impossible, just time consuming.
I completly agree that most people will like the way you have yours set up better than side firing. The majority of people will never even try, or know how to properly blend the sub with the front stage. And there is nothing wrong with this, there are times when I prefer this type of set up too. A true SQ set up will blend seamlessly with the front to the point where you can hardly tell there is a sub there, but this is not what a lot of people want, they just want some bump. I just wanted to let the original poster know the diference, and clear up some of the issues of advice being given here.
The main thing to remember with audio is, all the advice in the world is no substitute for just trying. What sound great to one guy will sound like crap to another, you just need to listen for your self and set your system up how you like it.