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Check out the remixes - there are a number of adaptations for different coins around the world.

Or you could try putting 3/4" (19mm) dia steel balls in the coin slots. I haven't tested this, though, so it may not work.

I decided to use coins because they are much more consistent than washers and they have a better packing density than spheres. And if you are in the right country, they are more commonly available than either.