Here's a hint that I got from Tim, AG4RZ:
If you're double-boxing or just want to make conformal packing, that foam-in-place insulation that comes in a spray can is great. I would imagine that, if you were to shoot a wad of that stuff between your inner and outer box, the shipment recipient would have a hard time discarding the outer box without leaving evidence.
One other trick that I've used, when I used to do shipping as a job, was to stick a couple of lables on the inner box which read "inner box--- do not accept for shipment if visible."
73 de tom n4ioz