Adapted my 20x20x1 air filter return to a 18x18x4. The only problem was the old grate restricted air and it caused a whistle. I replaced the grate with a newer one and the whistle went away. The whistle was because more air was moving through the new 4 inch filter than the 1 inch filter. Better for the furnace motor.
This is exactly what you expect it to be. I ordered from two other companies first that sent their version of this product, and they didn't fit exactly. This is genuine and I was so happy that it fit and worked perfectly.