Heater box flap foam started to disintegrate. Any way to re-enforce it?

[Konstantine Bogach]

Gents,
I am changing leaking heater core on S4 '93.  The foam on one flap 
started to disintegrate.   The surface on once side is eroded. Little 
pieces of foam did flew out of vents.   Any advise on re-enforcing the 
flap?   Getting it from a dealer would be ideal but it will take time (I 
hope they are still available)  and I am kind of in rush to get the car 
running.

Thank you.
Konstantine Bogach.