When my heater core blew out a few years ago and I had to replace, I cleaned up the entire box. Those soft pads pretty much just brushed off of the flaps - they looked like some kind of thin foam..
Now I have pretty considerable air leakage past the flaps when theyre closed. Too hot!
Whatever goes back on has to be able to hold up to the heat.
![Hmmm :hmm:](./images/smilies/icon_hm.gif)