Help with box for 18" fi ssd fully loaded

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My buddy is trying to put a 18" fi ssd fully loaded in the back of his eagle summit van.. He has about 1200 watt rms of power to run the sub.. There is two different set of max dimensions we can use ( it all depends on how tall the box will actually be. The first set is a tall narrow box and our max dimensions are 27 1/4 tall X 39 wide X 15 Deep. And the other set is 20 1/2 deep X 39 wide X 21 tall. Any help would be wonderful... Would really like start build on box as soon as we can.. Also I would like it ported wit a tune right around 32 hz. And I not sure wat would sound best wit the speaker direction and port direction so any advice is wonderful.

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Any Help from anyone :)

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I noticed I should of put this on fi forum sorry