I finally found a decent heater/defroster plenum. The only issue is a small hole the the back side which isn't supposed to be there. Could install it as is because it would be hard to see but wondering is there a way to fix it? I remember reading back in the late eighties that a way to make this sort of cardboard piece to last was to coat it with a few applications of shellac. I would think that this would dramatically change the appearance. Is there a way to clean this material?