Thanks for idea but I think velcro is the worst possible option . It would prevent the box from sliding around but it would allow shifting/vibration due to the imperfections on the road and would decrease the bass output. I would rather opt for the MDF/plywood under the carpet and mount to that.

Now I realize the gas tank is in the hump/wall or whatever you want to call it. I was talking about the front half of the floor of the trunk. You know when you lift up the carpet, the rear half is Styrofoam material with the jack, air pump etc. and the front half is just metal covered by a thin sheet of something. I would like to drill into that metal towards the front of the trunk floor.