Question....What is preventing the south edge of the box from sliding into the gap?
It appears to me that it is touching the box below the south edge such that box could be supporting it. However, it also appears to be overhanging less than half way of the north/south dimension of the upper box. Therefore, depending upon the weight distribution of the contents of the box, it could be that the support from the lower box is not even necessary due to a cantilever. Either way, it is being supported adequately to prevent it from sliding into the gap.