When we bought the house, the furnace/AC air filter was just sitting in the housing. At one time they had put tape over it to keep it from sucking air in around the sides instead of the return. Apparently, pre-made doors are impossible to find, so fabricating one myself was the only option.
This door is NOT designed for looks, it was made for function. Please note that you can fashion a door out of other non flammable materials, even duct tape. Sheet metal was the easiest and most available for me.