This is another fun thing to do with Lego terrain:
The build is very easy. The friction of the normal Lego connection makes the plate pop out when you press the axle swiftly.
It is easy to mass produce these things. Your players won't be surprised after the first one, but if you use them as your normal dungeon wall segments a lot, without triggering the trap, then they get used to it. Then you can use it in the middle of a combat encounter or some other situation when they are focused on something else in the map.