Ideally, if you do not publicize the App ID / appspot domain, it is not likely to be known & requested. Anyway, the simplest way to do this will be to redirect

Ideally, if you do not publicize the App ID / appspot domain, it is not likely to be known & requested. Anyway, the simplest way to do this will be to redirect requests received on the appspot domain to your custom domain, from within your code.