I used to have the same problem but it's fixed now. I used resistance bands (the kind you use to exercise). The velcro on the exercise band for your hand is tied around the rear metal upper frame inside the trunk, and the whole band is wrapped around the subwoofer box between the subs. I use velcro underneath too, and it helps, but it will do a lot more once the box is no longer rocking back and forth. Be very careful not to damage your spare if you use L brackets as the screws can puncture it.