What drivers are you using ? Did you consider an infinite baffle design ? Basically the drivers "see" the cabin on one side of the cone and the trunk on the other side. Undoubtedly the most neutral sounding bass with the fastest attach, at a cost of efficiency and low end extension. Extension is rarely ever a problem in a sedan with two 10's. The bass reinforcement of the small environment makes good for the earlier bass roll-off. Efficiency can be made up for with amplifier power (as long as the drivers can handle it). My suggestion would be infinite baffle with an ajustable Q active highpass filter section in the bass chain (pre amplifier) to provide bass equalization. Easy to build, too (no cabinet). You just need to find a slick way to mount the drivers.
Good luck whatever you do !