Abstract: An apparatus for covering a top of an open container is provided that includes a cover, an arm assembly, and a lock mechanism. The arm assembly can move between a stowed position in which the container top is substantially open to an extended position in which the container top is substantially covered. The lock mechanism can easily and automatically lock the arm assembly in the extended position as part of the container covering operation. The lock mechanism may include a latch and a latch block that cooperate to automatically and mechanically engage and retain a hook mechanism. The lock mechanism may also include an actuator that, when energized, permits the latch to release the hook mechanism.