Thanks Fright. Here's a long shot of the back. You can see the spring is attached at the top of the cylinder and screwed into the coffin below it.
This is a gate spring. The type that come with gate hardware.
Here's a closer shot...
I used PVC (Schedule 40)for the whole thing including the attachment to the air ram. The rod had threading at the end, I used a tap to thread a hole into the PVC and threaded the rod into it. I just stuck a piece of round aluminum stock slightly bent to attach the two pieces of PVC you see here. I would recommend a bolt and locknut, but this held up for maybe 1000+ activations.
This is a picture of the inside, I just stapled the pants in the coffin and you can see the simple pivot joint. The frame is also PVC and that is what you hear slamming into the coffin when it returns to resting position.
I really didn't put much work into this. But it was one of my most loved/feared props. I just propped it into place at the standing angle it is in the video. I like that better than laying the coffin flat. It did send 'em running. I'm smiling just thinking about it! A retired school teacher operated this prop for all three showings, I think he considered this revenge! He sure did enjoy himself!