OK. For a simple rectangular box.
Width - 33"
Heigth - 15"
Depth - 12"
These are external dimensions. Use 3/4" MDF. This will put your box volume in the upper range of the Kenwood recommendations for that sub (after displacement, near 3cuft.). I would suggest placing the box against the back of your rear seats with the subs facing backwards but you can experiment and see what you think sounds best.
Good luck.
Carpe poon