Mine was rattling pretty good too until I took a closer look at how it was secured.

There's a small metal bracket which looks like a hook. It's what the box clamps onto when it's locked to the luggage frame. This bracket bolts to the frame with two allen screws and 10mm nuts on the back. I loosened these screws and moved the bracket a few mm forward. It's take a little more pressure to clamp, but its a much tighter fit. No more rattling!