That will work. The one thing I would add to it is some bracing at either end to keep the rails perfectly upright. You can add on to the jig and it will rout straight lines for you. If you want to go parallel with the rails then clamp onto your cross arms on either side of the rails and if you want to go cross ways then clamp blocks on the rails so that the cross arms can only go back and forth.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.