[Rugorim]

Yes, the trapped boxes have to be locked for the trap to trigger. Once locked you either have to break it with an axe or use a lockpick to open it for the trap to cause damage.

So, the best choice would be to fill the box with some ore, trap it, lock it, and place in designated spot. The box will still be able to move, but with enough pounds of resources it wont go in their pack.