Funny, I was just thinking about this today and I thought to myself "myself, how am I going to do this?". I figure, I can take an unwanted contact sheet and cut around a 35mm neg that's a little bit bigger cause I usually like black borders. Then measure the rest of the space, cut it out, and stick it to the bottom of the part of the glass. I figure, I can do this with 6x6 and 6x9 formats that I have. I think it may droop a little and I think it'll get tedious. But you are right, there has to be way right?