I recently had an epiphany. I was trying to hack a method to hold the film flat and away from the iPad, and was banging my head against the issue. 35mm was solved because I have a Pentax bellows with slide copier, but my medium format was suffering. I seriously considered buying a used Pentax 67 bellows with its 6x7 slide copier, and a 67 to K mount adapter, but that setup is VERY expensive.
Then I realized--I have an enlarger that I'm trying to get setup to do wet prints. That enlarger has a bunch of negative holders (at the moment, 4x5, 6x7, 6x6, 6x4.5, 35mm, and one with a small square opening, I'm guessing for 126 film, but I don't know.) It turns out that these are perfect for holding the negatives flat (go figure), and the thickness of the bottom plate is sufficient that I don't see any grid when shooting at ƒ8. So now I'll use those negative carriers for digitizing and for enlarging. And if you happen to need something similar, some of the negative carriers cam be had pretty cheap.
This dark shot would be one of the worst if the grid was visible because all that black would have been transparent: