box suggestions

I got a kicker 2008 L7 solobaric 15" sub, and a good amp to power it.  I was just curious on what kind of dimentions i should make the box (2003 ford escape, so room isnt a issue).

A buddy and I came up with a box that is 20x20x25 (width*height*length) it has 4.6 cubic feet air space, and im thinking of putting a port, not sure what freq. to do it at though.. 

i did the calc on the site for a 18.5 inch width and 3 inch height port (at the base of the box), it needs to be 9.7" for a freq. of 45 hertz.  Just not sure if i should look into changeing the hertz for better sound or not. 

I havent built the box so if you guys have any good suggestions on what to build just throw me some ideas.

thanks for your input..