My father would mount them in glass and used a black tape around the edges and he also had Bausch & Lomb 2 1/4" x 2 1/4" metal frames that held the glass.

He had a great Bausch & Lomb projector that took slide trays. We lost track of his projector and I have not been able to find one anywhere.

Since Kodak started walking away from slides the last few years I have not bothered to look for the projectors.