well i had my box built and when they did it they put that crap in it and said since its a small sealed box it'll make the sub think the box is as big as the original box i had it in(the same size as my boot space in the X), as for carpeted box, its juss looks better than normal wood, not to mention you can cover up any stuff ups during construction of the box as well as screws.

the carpet is only for looks, in my old car i just had a wood box, then one day put carpet around it... no difference at all... just looked more respectable