Then you might have to work out other means such as EMI or metalic coatings on the "assumed plastic box" . I used the terms EMI not because of the radiation, but the metalicness of it. One would hope that the person holding the device is at the same potential of what's being plugged din. You may be addressing the wrong dot.
In general, only the sourcing device gets the shield, but it better be connected to Earth. USB does have bidirectional data paths. Using the reason is to remove ground loops, but if you can make the devices the same potential your good,
For fun, wrap the device in aluminum foil to the USB shield and see what happens. An ersatz metalic box.