need help bad!

ok im building a downfiring box for under my rear seat in my 2006 silverado. i need port and tuning help. i want the maximum output and quality sound i can get. my box is a lil small for the sub but its all i caan fit under the seat even after rasing it an inch. here is what i got so far.

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i need to know how big and the best place to put my port. this is what the sub calls for 2 cubes - 2.5 cubes, 20 - 30 Inches of Port, 28-35 Hz