port dimensions for a box

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Need help detemining port dimensions, want to stick to a round or squared port. Box measurements are 26Hx18Dx48W, im dropping a pair of 15`s on the box. Info of each sub is as follows:

f(s) 35.66

Q(ms) 7.27

Q(es) 0.5927

Q(ts) 0.829

V(as) 56.730 liters

(2.003 cubic feet)

Any help is appreciated.

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The box is going in a sedan replacing the back seats. I wanted the subs facing up and port facing the front windshield. Wood thickness is .75, the measuremnts i posted are total measurements-tip to tip ,not air space or usable volume space. Thanks for any help!