I'm gearing up to do some contact printing, and since I like the "white borders" look, a quick search has shown that a lot of you are using tape of some sort (rubylith, silver mylar, etc.) to create a window right on top of your contact frame's glass.

The question I have: since I hope to print different sized negatives (e.g. 4x10, 5x8, 8x8) in addition to "native" 8x10, would each format require its own separate, full-sized replacement glass that fits inside the contact frame? Or, if the original contact frame glass has been masked for the biggest negative (8x10) I'll be working with, could I simply lay a second piece of glass that's been taped for a smaller format on top? (Full disclosure: I already have some 8"x10" panes of anti-glare and museum glass lying around from another project.)

Any thoughts/alternatives appreciated--I'm suspecting that there might be some diffusion issues that make the edges not quite as tidy, but hopefully y'all will be able to point me in the right direction if that turns out to be the case.