Was getting tired of the rattling, but didn't feel like tearing the rear of the car apart.

I've read up thoroughly on the bolt mod, the tennis ball mod, and the stuff it with foam mod.

I opted for the styrofoam cone mod. Picked three of these babies up at Michael's for a few bucks.

Dropped the sub and wedged three of these puppies in there-- one on the outboard side of each speaker hole, and the third up against the tray that sits up against the rear window.

Happy to report that the rattle is completely gone with no negative effect on sound as I didn't have to fill that acoustic cavity up with anything.

Total DIY time was about five minutes.