diplomatic bag

In actual practice, diplomatic bags are indeed used for exactly this purpose.

Perhaps we could smuggle him out in the diplomatic bag.

Both went into the diplomatic bag for secure shipment to Washington.

Get it to me in the next diplomatic bag?

By eight that morning the diplomatic bag was in the State Department.

The film and a copy of his story, he decided, would find their way into the diplomatic bag.

Santos won't be coming to England any more with his diplomatic bag.

What the hell, the diplomatic bag goes over in eight or nine hours.

No, this one would get written up, encrypted, and put in the diplomatic bag.

A diplomatic bag must never be opened even on suspicion of abuse.