xylophone keys

I’m probably one of the few people who buys a xylophone so I can take it all apart :). Here’s my plan. Bob is going to build for me a wall mount where I can mount a series of xylophone keys in descending steps. We start a marble down the steps. The marble plays the first part of Frere Jacques. As it gets to the end of part 1, a second marble starts down. While the second marble is playing the Frere Jacques part, the first marble is now playing “Dormez-vous” and so on. They play the round together. Easy, right? 🙂 It’s for a “motion” art show coming up. And then I just need a little chain-system to let the marbles get back up to the top again … I wish the keys were smaller, though. I don’t want this setup to be too deep front-to-back. Ideas?