Ed - the pictures are from my dash pad replacement project several years ago. I didn't even understand what I was looking at when I was messing with this trash bag stuff. It was mostly still attached, so I shook the loosest stuff out of it, sprayed some 3M trim adhesive along the edges where it was loose and stuck it back up there.
If I had realized what I had, and how unusual it was to still find it there, I might have done something different - like the template you suggested. Maybe next time it's apart!