It is a simple arrangement. Imagine that we have an empty cubic box. Outside the box the electric potential is infinite. Imagine that you put an electric charge at the geomectric center of the box. What is the field around this charge? In other words, the tool I have to calculate the field generated by a charge is the Coulomb law, and according to this law the only parameters that influence the field are the distance to the source and its charge, but I guess the shape of the box must have an influence too. Let me know your thoughts.