The centering part is the part that we have to brainstorm. Cause I can see it now when you place the neg down and the glass, the neg always seems to move a little.
Perhaps (just thinking and typing), you take that matt paper idea, put it over an 8x10. Place neg in openning, and somehow, tack the corners. Then remove the matt, and place glass. I wonder....will using clear scotch tape be seen on the final output? Won't light go through it like glass?