With a large radius like that, I doubt that you'd even see a gap at the edges. But if you do, and it bothers you, there are a couple of approaches. One is a thin rubber washer which exactly fits the profile of the button (probably have to make them yourself), and the other is to use a forstner or spade bit, slightly larger than the outside dimension of the bezel, to create a very shallow pocket. The latter is a bit risky without the proper tools.
Personally, I'd just live with the tiny gap at the edge, if it's even noticeable.