Pardon my ignorance, but what would happen if you just divided the hold into smaller compartments? That way sloshing would only move things slightly to one side.
It's already divided into smaller compartments, through the containers themselves.
Think of a ship transporting prisoners in their own little cells. Then think of all the prisoners working together to throw their weight against the walls of the cells at just the right moment in an effort to sink the ship.
The prisoners would only have enough of an effect to make the guards a little nervous. The ore sloshing around in the containers weighs tons and tons, though, enough to actually sink the ship.
