I have a 10" subwoofer in a small enclosure in there along with auxiliary fuse box powering all non-critical devices (LED lights, air compressor, 12v outlets, water pump, etc...).
The lockable door is kind of useless when you can just slide the seat forward and reach in from the top but thieves are generally not that adaptive. The lock is probably a decent deterrent, if nothing else. I put velcro around the edges of the foam cover since the subwoofer made it vibrate and make noise. This makes it very, very difficult to remove without taking the seat off. You could always make a similar plastic cover and bolt it down for added security using rivnuts.
Before I decided to put the subwoofer in there, my plan was to fab a small cubby box that was accessible from the side-door to hold random things. I think that would be a great use of the space.