I don’t think you could lock the box shut, but that would be a question for rules@pd. Remember that the objects you’ve sent aren’t in the box (either IC or OC), so it’s quite hard to steal them, although obviously if you aren’t around someone could take the trade on your behalf after the offer is returned.
You could chain it to the ground, though. That is probably fine. Or just cast before a sequence of nice big rituals so you have something to watch and people to talk to while you wait. Or buy a potion or something to get a rank of lore so you can cast it in your own tent where it’s nice and safe.