I like both the moving crate with the chain and the door idea.

If you do a door, (if you can,) put an old hasp and eye type of latch with the padlock thru it. Have a blue or green light shining under the door and maybe some spooky noise, and scare inside.

You can do similar things with the crate... light seen thru cracks and noises.

Great theme!

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Spectorized