Hi again! An update on our previous conversation.
Since you were interested on a security measure for the backseat. I installed a magnet held: battery pack/vibration sensitive alarm/GPS tracker into the box. They all use those small light STRONG magnet name-tags (from employees at stores like Target, Best Buy, etc.) to stay connected to the walls of the storage box. After all 3 items were installed, they really didn't take up much space, as you can see, the U-lock still has plenty of space to hold more chains or other locks.
Btw, those small holes drilled into the sides of the box were to allow the sound to come out louder. After much testing, there was a huge difference when the box was completely open to when it's locked close. Those holes helped a bit. My friends thought I went a little overboard with the GPS locator, but I think it's worth it to protect my investment.
Love the ingenuity, and don't think he went overboard at all. Maybe we'll turn this into a product some day!