This idea was to be apart of the Hex House a few years back. I built the pneumatic workings for two of them but ran out of supplies to build shelves and there were no trunks left. So, instead of using this idea, I put cylinders into old luggage and hung those to the wall. It was horrible, but they still got scares.
The concept was that a trunk up on top of a shelf in a closet would tip over and hang open, over patrons, letting a ghoul drop out. Then it would go back for the next people to come through. The closet was a creation of someone else at the Hex House and he called it a 'claustrophobia' closet. It was just very close walls making people walk single file.
The workings are simple one cylinder to make the trunk tip, then another to open the lid. The monster is just tacked to the lid and bottom with a spring on his back attached to the lid. When the truck closes the ghoul is pulled back in.
I hope you guys use this, it was an idea I regret not being able to complete.
-also, its a simple idea that I am sure most of you have already done, I just haven't seen it here yet.