This is an interesting thread cause I've been thinking THE SAME EXACT THING for about a year now. I've just only played with ideas in my head and perhaps, together, we can get somewhere. I like the idea of creating a mask with thin opaque or better yet, wit that rubylith right on the glass. But I was thinking the same, the thickness of the mask will ruin the pressure. Perhaps, slightly bigger piece of paper under the 5x7 and under the 8x10 so that it gets raised a little so the glass will make contact?
I was also thinking of making the "hole" in the glass by taping the section (in my case, 4x5 negs) on top of the glass and carefully position the neg under that "window".
Or now, that I reread my reply, you can take the rubylith and make that the TOP portion of the glass.