Originally Posted by Whiston

You could toss your subs parameters into some enclosure design software, then adjust the volume a decent chunk either way and determine what your range of acceptable output is. You'll find most enclosures can be fairly forgiving. A sub that asks for. 75 cubes sealed may do quite well in anything from. 5 - 1.25 and possibly even more. Don't be discouraged because someone says your sub needs exactly x amount of airspace, it's more like x amount +/- 20% or so. Go with a ported enclosure, and have even more leniency with your airspace. Chances are, if you're good with fiberglass, you'll come out with an exceptional product anyways.