I scoured what I thought was every corner of Bonneton before I gave up and consulted a guide to find this Moon. It's not difficult to get, but it is one that requires a good eye.
In a stage on the back of Top Hat Tower, you must capture a ParaGoomba and fly across waves of poison, grabbing Moon pieces on the way, to reach the exit. However, flying under the farthest platform and through the broken fence will lead you to a second Moon, hidden behind a high wall!
When I played this stage the first time I didn't even think to look beneath the platform at the fence, and probably wouldn't have spotted it if it weren't for the Internet.