As a house cleaner I get asked quite often if I could clean these cold air returns. They are huge magnets for dust and dander being that they are so low to the floor.
No disrespect to my customers, but my answer is always "no".
Not because I'm unwilling to (per say) but this is typically not a great use of my time. There is a much easier and less labor intensive way to do it. Simply place them in the dishwasher, and put through a regular cleaning cycle.
If you don't have a dishwasher, soak in hot water and a bit of dawn dishwashing liquid in the bath tub for 30 mins, and rinse.