What vehicle?

You'd have to air-tight the trunk and even then the trunk lid wouldn't be sealed completely as it vibrates and you have small air leaks all over. If you used rubber seals with a tight lock it'd probably work. 4 sealed 10's could be used in around 3 cubes but I doubt there'd be much low end with that little airspace. There would be too-much space free air anyways. If using a trunk with folding seats, your best option to fit a larger box is to cut the trunk-to-cabin opening to the maximum L & W dimensions, bracing the trunk with maybe .25" of steel welded to each side, then get the box in through the car door. There's a bit more cone area with 2 sealed 15's (+efficiency) so if you could fit 6 cubes that'd be better. I want to cut my trunk opening for 2 18's sealed in 10 cubes.

 

I say you try an 18''. mine sounds sick for a daily setup and still gets loud. the only thing that sounds bad is the dome light because its rattles too much. I think theres one for sale in the classifieds so you might be in luck...