Enlarge the hole to about 7 inches or whatever the cone diameter of the sub is. Then mount the box with the driver firing into the hole. THe ported box suggestion would probably be pretty good except that it requires a much bigger box.
I wonder how much depth we have on our decklids? When i get motivated enough to go outside, i'll measure it. What i'm thinking is if we have 12inches of depth, you could get some sonotube, cut it in half and use it for the sub enclosure. It would be a very lightweight and strong enclosure that would be cheap and unobtrusive. To get .5 cubic feet, you would need an enclosure that would be the same size as a 12" cylinder 15 inches long that would be cut in half.
Dave
