OP why are you going to throw a bunch of subs in a trunk if you know it is not ideal for the woofers? By your thinking, why not just get like 8 18" and hook them up free air? Its not ideal but should work. More subs does not always equal louder! Look at how much space you have and what would work the best. If I were you I would listen to some of the recommendations.
Unless you just want to impress the middle school girls by how much money you've blown and how loud they perceive you to be, step back, evaluate the situation, consider your options, come up with a game plan, then proceed.