Sorry, I cannot find any descriptive picture for this, I'll see if I can take a clear picture tonight, but here is hopefully a better description:
1. The "door" on top, I think serves dual purposes: both an access door and an escape hatch. It is about 8"x10" wide, flipping open like the trap door except it opens out. A rot cord is attached to this door. a bungy cord is attached to the rot cord, a hook is attached to the bungy cord. You use the hook to keep the door closed. Think of the rubber band that keeps the trap door closed at packaging. My concern was, when the rot cord is rotten, the hook is released so the door is free. However, since it opens out, the crabs need to push it from underneath to escape. If it were to be installed from the underside, it would drop open giving the dumber crabs a better chance.
2. This is even cheaper than the Danielson (a little more compact, 21"x21", seems a bit studier too)