What kind of latch mechanism should I use for this movable ceiling under my deck?
I would look at seeing if you could use large size slider bolts. I used this type of device to provide the safety latch for a large hinged stairway I had built in my garage some 13 years ago. The stairway was raised and lowered via an electric winch and cable/pulley system but I added two of the slide bolts (one on each side) as a safety measure when the stairway was in the hoisted up position. The ones that I used had round bolts that were 5/8" in diameter had a slide throw distance of about 3". They were plated with a typical hardware silver color. 

You may want to consider something like these power coated sliders for your application outdoors. 



In addition I want to suggest that you also consider a secondary safety mechanism to prevent injury and property damage in the case that the sliders are not kept latched fully. This could consist of some short chains that are lagged to the deck support beam and engage the hinged section via a hook on the side or end. These chains could hang loose when the slide bolts are in place.