Heres the thing, You cant just make a box, w/ a mystery length/diam port into the shelf. It may tune the box to a high freq regardless of what the x-over is set to allow. In 99.9% of car systems, sealed small boxes are most musical and user friendly. Ported boxes are BIGGER, not smaller, harder to tune, and much more prone to builder error. Also, Box volumes are according to your sub, not just size, but the brand, model and # of voice coils. When you buy subs, ask alot of questions, be honest w/ the dude at the shop what you enjoy listening to (even if you are a closet rap guy like me) and allow them to lead you to what you need. Theres no reason a good pair of 10"s in a sealed box w/ a few hundred watts of REAL power cant please you....even if you like Nelly. I had 2 10"s doin almost 140db....(1000 watts on them though) thats all you'd ever want or need.
Also, generally speaking, subs sound better facing the rear to allow the wavelength to make your ears hear the subs sweet spot of design. Low bass freq need a longer path to make accurate sound, and nothing sounds worse than a sub woofer cone,(maybe port noise) so allow some room. Also SPL can rise w/ deflection, but so can rattles.
