I took a T bracket and screwed it in to the bottom of my sub box. The bracket goes out into the utili-track area. there I took a bolt and a coulple washers inserted the bolt from the bottom and tightened it up on top with a wing nut.
it looks like this with the bolt running up the bottom.
wing nut
washer
bracket that is hooked to the box
washer
utilitrack rails
washer
lock washer
bolt head
I have yet to figure out how to secure the top down, so I use a bungee cord from the bar on the back of the seat to the little hook next to the power source in the back.
You can kind of see what I am talking about here. It's not the best but I need it to be removable and haven't thought of a better idea yet. However, it doesn't move.