As far as  the black shield showing up in a reflection, that can be a problem. I once did a render with a bunch of reflective spheres and after using the shield to do a spot render, everything looked fine. I finished postwork, then noticed each sphere had a tiny black square reflecting in it. You could duplicate the shield and make the one facing the scene reflective itself. Also, you might get a shadow from the shield in some cases. I don't even know if you can turn cast shadows off on an object, in Carrara.