You must be a great deal more skilled than I am when focussing with a loupe.
All my current cameras have a protective sheet of plain glass over the fresnel. All of them have the same arrangement of the focus screen assembly too. That is, from the lens side: focus screen, with the ground side facing the lens, fresnel, then the cover glass.
They are all comparatively new camaras.
Adding a cover glass (or a thick fresnel, perhaps) means you may have to re-focus your loupe.
All my current cameras have a protective sheet of plain glass over the fresnel. All of them have the same arrangement of the focus screen assembly too. That is, from the lens side: focus screen, with the ground side facing the lens, fresnel, then the cover glass.
They are all comparatively new camaras.
Adding a cover glass (or a thick fresnel, perhaps) means you may have to re-focus your loupe.