Building a good box depends entirely on the speaker units that will be used in it. Each speaker will have its own specifications that will determine the optimum volume (air capacity) of the box. Given that you know these specifications for your subs, you may be able to construct one using speaker box building software.
I would definitely advise using either a passive (ok) or active (better) low-pass filter for the signals you are feeding to your subs - they are intended to reproduce only the very lowest range of audible frequencies (<200Hz) and won't work at all well if they are fed a full range signal.
valuable info here:
http://www.caraudiohelp.com/custom_c...udio_boxes.htm
speakerbox design resource here:
http://www.ajdesigner.com/