Without the upholstery, there is obviously the foam seat cushion and a metal bracket. In order to fit the sub box, you have to alter the frame bracket. You have to cut out some supporting bars with a grinder and then the box is the same shape as the frame, fits right inside the frame. The box comes with a bunch of attachments that holds it to the frame, it is actually very sturdy. Then you have to remove a few inches of foam from the cushion in order to fit the box into the seat covering. I actually have some pictures of the whole process, so when I get home from work tonight, I will post the pictures of the process basically step by step.