Acetate sheets for use on Over-Head Projectors (OHP), used to come on a plain white backing sheet. The top was glossy smooth, and the back was a little tacky; if you can find those ...

Also curious to know if you can get slightly better color spread by looking at it from far away; I assumed the misaligned light or distortion through the glass might be enough to throw things off if you're working with tolerances as small as it looks.

Or, maybe the electron beam scatters more as it's deflected to hit the the outer pixels, possibly giving you a signal too muddy for your magic potato pixels to work with. (these are obviously just vague guesses, I didn't even know this was possible before today)