Rather than making a square opening in the darkslide, you might consider just cutting off the end of a darkslide, leaving only about 2 3/8 inch of the original darkslide. Then all you have to do is mask off an inch on the one end of the GG.
Cutting a hole in the darkslide would leave a very fragile darkslide.
I have done similar things to 8x10 darkslides to get two 4x10 negs on an 8x10 sheet of film. As with the image below. Fortunately, my GG is gridded, and I just use the gridlines to compose. The modified darkslides are close enough to the film to be fairly accurate.
PS -- I prefer to mask the negative so that I have a rebate around the image for contact printing.
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