Thats a rubbish fix! I sold my 3ds due to it not being portable, couldnt carry it around without it damaging itself. Used to have to keep a cloth between the screens. I dont want to stick bits of tape to it either, more dust will get in the less closed the lid is. I keep my eye on this problem as Im looking forward to owning a 3ds again if I can get a decently made one. I know it doesnt affect all units but its a gamble
A couple of those foam circular feet thing work a treat on my newly replaced one(due to this issue, with the old 3DS), I just stuck 1 each side of the top screen, where the bottom screens verticals are, and no issues yet, and you don't notice them.
I'm sure the feet came with my surround speakers, but they are really thin, and the lid closes just as it did before, but with extra cushiony goodness.
Also done it to my boy's 3DS too, so no more cloths inbetween screens.
The frame of the lower screen, the raised part, seems to touch, the upper screen when closed, or when some pressure is placed on the outside of the 3DS, resulting in it leaving marks on the upper screen, which over time, develop into not so much scratches, but stains, akin to something grating the screen.
My original one did it, my replacement I put foam feet that my surround speakers came with.
Since removed them as a tip I got seems to show mine isn't affected.
The foam that comes with the 3DS in between the screens, place in inbetween the screens, and close the 3DS, press on the top lid(with the dual cameras), and open the 3DS, if there are marks on it, with the foam in, it won't mark when they aren't in...weird, but it seems to work..both mine and my son's replacements mark with the foam, but don't mark without it, when pressure is placed on the lid.
Both our first ones did it, and his was really bad, so much so, the top screen had 2 vertical scratchmarks on it, ON the screen, which resulted in a sort of sparkle effect when playing, but give them their due, Nintendo replaced both of them, without too much of a fuss for 2 split new boxed ones.
His replacement, a metallic blue one, and mine, the black one, both seem free of this strange phenomenon.
But I do notice now, that when looking at the top screen, with it barely open, it seems sort of bevelled, away from the bottom screen.