You might try tuning the cab you already have. I posted a long-winded description of the procedure here http://www.guitartricks.com/forum/showthread.php?threadid=2309 if you're interested.

If you still want to build a new one, I recommend making a closed-back box with a front panel just big enough to comfortably mount your speaker(s), and deep enough front-to-back to hold the head. Then tune it using the techniques in my other post. Its surprising how much leeway you have with a closed-back cab.