The sub would be snug to the side of the boot area, if the floor wasn't a silly shape. It looks like it is taking up a lot of room in this picture I will admit, but I can still fit a pushchair in the boot with it with no problems, or a set of golf clubs.
The security feature I mentioned is called the Ghost, by a company called Autowatch. Even with the keys to the car, someone wouldn't be able to steal it. When the Ghost is installed you choose a combination of any of the buttons in your car as a sort of start-sequence. They need to then be pressed in that order each time, before the engine will start. Most cars these days are stolen with the keys, and this completely stops that from happening