Just use ReactDOM.render into a separate, top-level <div> that is usually empty. And then have the close button of the modal call ReactDOM.unmountComponentAtNode.

Alternatively, don't bother with all of this and just mount the modal into your children. Use CSS `position: absolute/fixed` to position it. Of course this assumes your ancestor elements don't have certain properties like `position: relative` or `transform` set, but this is usually not a problem.