To build something to suit your car... For a sealed box most 12" subs want about 1 cubic foot of air space for real punchy bass, or a bit more than that for slower, boomy bass. depends on your music preference! Too big and it sounds too boomy with no responce. You can fill the box if its too big to make it smaller - ie. pillow or special sound dedening material (stops internal sound wave cacellation).... Have fun!