What was the point of having a box for Batman if his goal was to never show those boxes to the others? I mean, if Batman crosses the line then who would know where to find his kryptonite to stop him?
Also, by now if Superman crosses the line then he could just go destroy those boxes or use them to stop the others. Well played, Batman. [sarcasm]
For a full proof system, I think Batman should've show those boxes to the others with a double of boxes located elsewhere that only Batman should know where and give boxes of his kryptonite to the others.