I've built a way larger than required box before. I did it and put more pieces of MDF in it gradually to see what size of box sounded best in the vehicle. Eventually, I decided on a size and cut part of the box off, making it go from 5.?? cubic feet, down to about 3.2 cubic feet. It overexcurted way too easily in larger than 4, and there wasn't any audible difference in the lowend of things between the largest size, and the 3.2 cuft size. Above 50hz, the largest size suffered alot. Bumping it down to 3.5 boosted it dramatically, and cut down on the "overexcursion without alot of sound" problem. Then, cutting it down even further to 3.2 cuft, there was minimal audible lowend loss, so thats where it stayed. The box still sits at that size. Thats the first box I built. It housed a Kicker C18C, which is no longer made. Pretty fun if you are bored enough.