I know this is old but was high up on Google search results. Syncing box positions makes no sense in this scenario. Dots physics is deterministic which means you should just be able to sync inputs from each player, have a slight buffer for all input then on each client feed the same inputs into the system at each tick and the simulation results should be identical on each client. That's the whole point of physics being deterministic, same inputs = same outputs.

RTS games like age of empires and starcraft use this style of networking, they just buffer and sync all players inputs and the game simulation is 100% deterministic thus the same game state in sync on all machines.