I originally thought the same thing, until the gurus on here thought a horn would provide more output. And since its going under a deck, no reason to dig a big hole in the ground. .
That being said a large concrete bunker facing up with a cast lid suspended above the driver would also work. I can't imagine if a concrete bunker is shaped like the hole, but you never know. Any Horn guys like my idea of the under deck subwoofers?