I think based on Viking's comments he already has a specific box volume. He just wants to convert to a more complex shape while maintaining the exact same displacement.
That's really easy to do with concise tolerances. No program required. And yes, the math should work out to a dime, or two cents, or whatever.
The subwoofer sealed enclosure target is somewhat subjective to begin with. The woofer/enclosure combination is a tuned resonator. You can aim narrow with a bit more peak output or you can aim broad with a bit more linear output. Either extreme begins to give something up. +/- 5% of the optimum isn't a major thing to most.