Blue Pure 121 Light Gray Washable Pre-Filter, Removes Pollen, Dust, Pet Dander and Other Airborne Pollutants, Lunar Rock, by Blueair

See above. I chose the light gray, but other colors are available.

I actually bought this for a different unit, hoping that based on dimensions, it would work. It does! I love that I can pick other colors and that it helps keep the dog fur and bigger particles out of the filter. Only negative is I think I was sent the wrong color, otherwise the Lunar Rock color is insanely dark (super dark gray, almost black). Pretty sure I was sent the darker grayish color. This is likely due to Amazon making a mistake, not the company. Other than that, highly recommend.

Tried off-brand version and they were about 1/2 inch too short, leaving either a gap at top or bottom - and that was STRETCHED. Bought OEM version and strangely, it's about 4-5 inches too tall. Can't imagine it's a defect. Decided against pulling excess fabric over top and folding into interior as it would obstruct filter flow; instead pulling underneath so the unit's feet now sit on the excess. These things aren't complicated so the whole thing strikes me as odd.

It makes your space gorgeous!

I love that I was just able to change the look of my air filter by switching from the blue to the gray fabric. It changes the look of that whole part of my room. I love it. This is one of the best parts of the Blue air filters.