Settings are populated by the configuration subsystem. This means users don't create the settings objects, instead they obtain a reference to them by calling Config.Current.GetSettings<TSettings>().

This is ugly but can be hidden from the user by accepting an Action to various methods, for example var engine = Engine.For<MyModel>((EngineSettings s) => s.StreamName="myname") ; or Config.Current.Configure<EngineSettings>(s => s.StreamName)