I bought some white poster board to divide the pixels up, and measured the width and height of inside the poster and its depth. I cut 6 row dividers and 9 column dividers for each frame, and then cut notches in each divider where they would intersect using a bandsaw.
I sprayed it with reflect all, which I found on amazon:
I also masked the frame and then sprayed the glass with krylon reflected glass, on the inside of the poster:
That stuff works pretty well, I did two or three coats, but I would use more if I had it. It also seems to rub off when it gets bumped, so perhaps I would use real frosted glass next time.