I got a Rockville RockGhost v2 for my Jeep Renegade. This is the first sub I have ever had and I am not an audio expert so I can't give an expert opinion, but I like it. Here is a picture in my Jeep which is a full size (16") spare and another with it placed in the Cobalt compact spare.
It fit in the compact spare when flipped upside down. It sticks up a little and I would need to figure out something for holding it down (stock bolt not long enough). The cover still sat over fine. I had to put the jack down by the battery. That would need a way to be secured (thinking velcro) so it does not bounce into the battery. I will give this sub a try for a while longer in my Jeep before actually buying one for the Cobalt. It would be an easy install wiring wise. You could use the high level input with the rear speaker wires already in the trunk. Power would be super easy with the battery right there.
The trunk has plenty of space for any number of other sub solutions.