Alpine Type R question

Hi guys,new here and hope someone can help me out. I purchased a 12" 2+2 Type R last weekend and built a slotport enclosere to Alpines spec's. It sounds decent but I still think it will perform better in a diff. enclosure. I scanned the box designs and found this one.

http://img34.imageshack.us/my.php?image=2cubes32hz30sqin7cb.png

It will be in a 06 Mazda 3 HB facing the rear of the car. Im pushing a Kenwood amp rated at 500x1 (so probably 350-400 watts true power?) and the sub is wired so the amp sees 4ohms.