Just posting as a help to BearTrap. After seeing his post on this, I made one. I had a buddy needing one, so I thought I'd try & see how it went.
The silver pieces in the center are nuts I welded-on to the center "rib" of the trap. They allow the trigger to pivot either direction.
The trigger is the "U" shaped piece which passes through those nuts.
At the very top left, you'll see a trap-dog I welded to the 1/4" rod which runs the majority of the cage & holds the doors open. This rod has 90* bends on each end to catch the edge of the door when locked into the open position.
The "U" shaped trigger is nothing more than a prop-rod under the dog. WHen the trigger is hit, it is knocked-out from under the dog (or whatever you want to use) and allows the doors to fall.
Hope you didn't mind BearTrap, and if I did something differently than you, please feel free to clarify.
Smitty