I suppose if you went with say, two small tube subwoofers you could not block off the spare tire well and have them on the side, bearing the same load and the Styrofoam.
I like this idea Chris thanks! I was thinking of building a fiberglass stealth box on one side, but you may have steered my mind in another direction. But, if you have no spare, why do you still carry the jack? To help other motorists? To lift one corner of your CX-5, spin one of your sweet TSWs and become hypnotized?! (lol)