Wait a minute. The trunk will be on the floor, with no place for the sound to come out of?

I was about to offer a couple of details when I realized that this should work REAL BAD unless the trunk itself is off the floor.

So let's say you lift the trunk off the floor a couple of inches, then my comments are worth writing. But if the trunk is down on the carpet, it won't make much difference whether you cut a hole in the bottom of the trunk at all. You'll just have a subwoofer inside a trunk, with no place for the sound to come out.

now, back to my comments --

I think QQQ's comment is good, but let me be a little more picky about what I think he means.

Don't cut a hole in the trunk big enough for the entire subwoofer cabinet, feet and all, to go through, and set the sub on the floor.

Cut a hole in the bottom of the trunk big enough so that you (this is where I realized the trunk is on the floor) can take the feet of the subwoofer and place it directly on the hole in the trunk. The trunk will have to be off the floor at least as far as the sub's feet put it so that sound can come out of it, otherwise you will just have a subwoofer in a box.

I have also placed subs in cabinets, with the woofer facing down and the sound coming out of the kick space. A single-piece A/C grille, that won't buzz because it is only one piece, covers the sound outlet.