Do you mean having a 4:3 gameplay box? I think you add it onto one of the elements that appear during gameplay whether it has something to do with the health-bar or something else.
Ah yes, that's the element I was trying to talk about. You just have to use photoshop or any other editing app and then layer the 4:3 gameplay box (or whatever you call it) and then replace the original scorebar-bg@2x