There is no better way to ensure optimum sound quality from a quality subwoofer, than enclosure design. Take a number of factors into consideration. To a point, the bigger the enclosure, the better. But it has to fit where you want it to go. The next overall design for sound quality of a subwoofer enclosure will be two of the largest speakers you need or want, installed in a large rectangular box. The two speaker chambers are separated by a small empty section of the box to keep the speakers from interfering with each other. The inside of the boxes will be completely airtight and have a layer of cotton batting to help dampen vibrations. A three inch diameter port in each speaker chamber will help increase the punch of the very lowest notes.
For more info, see 6 Things To Know About Car Subwoofer Speakers