The kits and cardboard WERE rare when I pulled MY 71 Challenger heater box about 6 years ago now. Mine looked like yours. I cleaned it all up and thought I'd just Google a heater box restoration kit. NOT. If I had a Camaro or Moostang I there were some crappy kits available but NOTHING for Mopar. Now I make all the Mopar heater box restoratiion kits in A/C and Non A/C from 63-76. Don't have the 69-7X C bodies though.

Funny, many people ask me about that "gap". In fact some have told me I don't know what I'm taking about when I tell them there is NOTHING that goes in that gap

PS I just had new clips sets made for the heater boxes too. It's amazing how much is being remade or can be restored by artisans like Jules above!