Why can't it just be some random hash? For me, a key benefit of a disposable mail account is not to have any identifiers including username.
Why not? 285e5c0452918bf77370c4a013317be4cf5e1ff690cc33a7346e837f59cdca58@foobarbaz.invalid is, I believe, a valid email address. If that's too long, you can always truncate it.