I have this on it: Abus 54TI/40HB63 B/DFNLI it fits just over it (fortunately the plastic gives a little at the front) but so far it has kept everything closed in every wind.

I had something like this on it:
Burg Wächter security combination lock
BUT: it broke after a few days, which is more due to the lock than the box. I think it's better to pull a steel cable with eyelets through it and fit a proper lock.