Im building a non-traditioanl vented box for my truck. I dont have any drawings of it as of now, but i will get some CAD drawings up tomorrow. Here is my question.

Subwoofer: Massive Audio CW12 (2 ohms,running ~900w rms)

I originally wanted around a 2.5-3.5 ft^3 enclosure, however, after getting my outline cuts done to match my seats, i have found that my new, partially built, box will be 4.815 ft^3 after driver displacement. This volume does not account for any area that the ports might take up. So will this box just be too big for my sub? I guess i could cut the box down some as i havent put anything together yet, im just worried that i wont hit any tones over 40 hz.

Here are my driver parameters:

QES: .733
QTS: .642
xmax: 20.5
BL: 11.9
VAS: 27.12

If i were to make my box smaller, about how many ft^3 should i be shooting for? I would like to have this box tuned between 28-30 hz. Thanks