You have answered your own question. Some very nice filter return grills are available, but you don't want that. SO you need a louver door. Available at any good lumber yard. They might have to order it, but can get it in a few days.
Once you determine how many square inches of return you need, you could cut the door. You could use a conventional return grill on each side for appearance. You would want to box in the cut internally. Assuming this is a hollow core door, you need to beef it up around the hole.