Well, it depends on how you're going to use the system. If you want loud music you could design the box with a little 3dB peak in ported. If not, and its only for SPL competitions, you could build a spl box with a large port and high tuningsfrequency. But it's not going to play music though.
Heres a link for a extreme SPL enclosure: http://www.caraudiomag.com/technical/0203cae_box2/
Heres a link that will tell you a bit about reflections in a car, and cancellation: http://www.installer.com/tech/aiming.html
Its good if you just want a bit more spl, or think you missing out on spl.
Heres some reading on gain from adding a nother woofer and more power: http://www.installer.com/tech/conearea.html