I would strongly advice you against using knob,its eideutig zweideutig.
"Ribbed packing/packaging foam" maybe, or s.th.along these lines

Thanks!

Egg crate foam
Eggcrate foam

Allgemein nennt man solche Schallabsorber "acoustic foam"

Ach so, als Verpackungsmaterial sind diese
Matten eigentlich viel zu schade und zu teuer.

As plastic Noppen packing is known as "blister packing" I would use the same term for this, i.e. blister foam