"not standing on something" is achieved by the consequence of gravity. So it's not an issue.
But the "nothing supports it from sides" is harder to figure out.
You could check if there's another box on the side, but you might end up with really unrealistic behavior (like two boxes hanging in the air side by side)
Maybe you could have the floor as a root, and transmit a boolean from bottom to top to check if something is supported... hmmm I'll think about it.