What about velcro? Not sure if it would get enough 'bite' on the trunk carpeting. If that would work, just use gratuitous amounts of it, staple it to the bottom of the sub box, and stick it down. I have a 12" Type R in a sealed by, weighs about 30-40 lbs. I have it sitting down in the recessed area, and it never moves...guess weight alone keeps it in place.