Does anybody know how to make a light sub-box so it wont slow my car down?

 

You might try fiberglass with an MDF baffle for the sub. The tirewell seems to be the most used place for this. There are lots of tutorials on the web about it and dont be afraid give it a try as long as you follow directions on the resin and just be careful not to get it on anything.

 

I'd also say fiberglass...but keep in mind that it can be a beeyotch to work with. Unfortunately, wood and plexiglass are typically pretty heavy, so fiberglass is probably your best bet.