You funny That schiit (esp. the foam) is a PITA to peel off. Always some left over to scrape.
I reckon @jerg looked at felt and foam and chose felt for the reason mentioned. I don't have them in front of me, but I was thinking the foam and thick felt were roughly the same thickness. Granted, the felt will compress/spread out more, but the having used the foam on the back vent mod, it will definitely compress, too. If it's not too thick, it seems like it would be the better of the two to enhance the contact/fill the gaps.