Cold air returns for non standard duct openings
Cold air returns perform the function of routing the air that has been depleted of it's warmth back to the furnace where it can be heated up again. And in houses with air conditioning the process is repeated in reverse. The cold air returns take air that has been circulating in a room and has absorbed heat from the surrounding air and the cold air return routes the air back to be cooled down again after which it is supplied to the living areas of the house once again.
Often cold air returns are found to be having very unusual dimensions. This can make it problematic for locating a replacment cold air return. At the site coldairreturns.com they not only have a wide variety of standard sizes, but many styles can be customized in a wide variety of sizes, and even shapes. Sometimes it is useful to have built in filtration and with many styles of cold air returns this is definitely possible.