I saw a diffuser grid for a much larger Neopixels board and thought, "I want one of those for my Unicorn HAT!", so I made one :)
Simply cover with a piece of paper or similar diffusing material (vinyl plastic also works well).
Warning: I found out the hard way that full power on the Unicorn HAT will warp PLA.
Note: The grid of pixels is not quite square, so this diffuser will only fit in 2 directions. If it doesn't fit first time, rotate it 90 degrees.
Designed with 0.8mm walls for printers with 0.4mm nozzles.