I think you should go one step further and after they reluctantly agree to put it in "the box", it proceeds to smash it with a hydraulic press clearly visible t

I think you should go one step further and after they reluctantly agree to put it in "the box", it proceeds to smash it with a hydraulic press clearly visible through a pane of glass on the front. Of course, you don't actually smash their phone, but a dummy you substitute with sleight of hand. Oh, but the look on their faces!