I had to strip out virtually the whole of my dashboard to rebuild the heater core in my 1970 280SL
I noticed that whenever you pulled anything apart the foam would disappear into dust...I tried finding suitable foam but could find nothing thin enough, I tried using the thin white packaging material, it was simply useless...Then I discovered something very simple, we have a local store that sells Velcro on a roll, its self adhesive. I simply used the soft side and it works perfectly
Here is one of my heater boxes, its such an easy thing to use