.7^3 = .7 Cubic Feet
I'd probably make the box a bit bigger than that, even if you're going with sealed....about 1 - 1.25 sounds about right to me for that sub. If trunk space is holding you back, .7 will still sound pretty good, you just won't get as much low-end extension as you would with a larger box.
Is there any particular reason you're making the box trapezoid? Is it for a truck? Here's some square box dimensions to start you out:
That comes up to .702 cubic feet (before speaker displacement). This design will take the most power, but you'll lose just about all frequency response below 40hz.
That one is 1.005 cubic feet (again, before displacement). This one will take up a little more cargo space, of course, but you'll be able to play notes in the 35-40 range loud and clear.
This is the big boy of the bunch at 1.266 cubic feet (before displacement). If it were my sub and install, I'd go with this one because it should show response all the way down to the lowest frequency the sub will play, which is probably about 25-30hz.
If you need more help, or are dead set on an oddly shaped box, you can plug your numbers into the calculator at http://www.eatel.net/~amptech/elecdisc/spboxnew2.htm
PS. All measurements given are outer dimensions and assuming that you're using .75" thick board, WHICH YOU SHOULD BE! Anything less is likely to break under extreme pressure.
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