I have been looking around but to no avail. You might need to get a custom job, like in the spare tire hole or up against the rear seats. I just had a L bracket with snap hooks on my last car to hold it in place. Worked like a champ.
It also depends on what kind of box you want, a regular sub box or a bandpass. Bandpass is usually heavier and doesn't bounce as much. I have seen people put blocks under the track that the seat goes on to raise that up and put a subwoofer under that. But you would have to do it to both sides to make it look assymetrical (i think that is the word).
I would stick with a regular sub box. Just make sure that when you build the box that it is close to what the manufacturer of the speaker says it should be in cubic ft. They are usually around 1cu ft. You could make one that is about that size out of either 1/2 or 3/4" MDF board and just carpet it yourself to match the trunk. That way it will be what you want it to be. You could either glue it together, screw it together or both. Then just mount it to the rear of the trunk, near the seats. I went with a dual 12" box that i made and it fits snug in the trunk near the seats, even when i drive like an idiot.