Heaterbox Pipe Passthrough - Reinforce vs Remold

That's gotta be the most beautiful heater box I've ever seen! Looks like you cut your own foam for the restoration. I took the easy way out and bought the kit from Detroit Muscle. Great service from the gal I spoke to on the phone. One of the pieces that came with it was a strip of white poly foam. The instruction sheet says it should be inserted under the metal panel in the back in order to help fill the space and close the gap where the cardboard and metal meet. I can't for the life of me figure out how that could work without blocking the flapper. I didn't take as many pictures as I should have during disassembly, but it doesn't seem like the cardboard will go under the metal. I was planning to just lay down a line of silicone sealant on top of the metal and around the edge of the cardboard. Any hints or tips?
I read all the responses and also have quit a bit of hands on with this kind of stuff already.
This box had the front ripped out where the J hook bracket was. (right above the door)
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