Why does my cell phone still work inside a metal box?
I'm not conversant in RF shielding, but it is possible that the simple friction fit of the cover on the box doesn't make a good RF shield. If it were that simple, microwave ovens would not require special RF shielding techniques to prevent microwave leakage from occurring where the inner perimeter of the door makes contact with the front surface of the oven. They would simply rely upon good (small gap) surface contact between the door and the oven.
Hope this helps.