a free air sub can not be put in a box and must be mounted so that when you look at the front, you wont be able to see or hear the rear.
i suggest that you put it face down on a large piece of timber or even a concrete slab, and then cover it with an acoustic deadening material like coarse gravel or river stones.
and then drive over it with a kenworth, or a mack at the very least.
sorry, i jest.
free air subs save you lots of space, and they are easy to fit..put a board across the boot behind the back seat and cut a sub size hole in it, and screw the sub on, but they sound really mushy and shitty, and they do not go well in a box.
if you havent bought it yet, dont.
if you have, play around with it and then sell it.