port length help

[05_ZX636 10+ year member]

the box is in a scion xa and is 21 cubic feet, wall built inside car to max size can get for 4 powerbass 3xl 15's. When I use winisd it tells me the port will be 16.75" long to tune at 30hz, and 40" H x 6" wide. But if im doing a center port with 2 walls instead of 1 port wall, how do I figure up how long to make the port?

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