Perhaps I'm lost.....that does happen to me....
I looked in your gallery and don't see speakers IN the archway.....I see some in the ceiling in-front of it. Maybe the pictures are still being uploaded?
Anyway, you first off don't
have to stuff anything in there. In-wall/ceiling speakers are engineered to work with or without stuffing or a cabinet.
If you want to use something, I'd go with the standard advice of poly-fill. Loose or wrapped? Um....I've always used loose....and since it's a "sealed" enclosure, no worries about poly flying out the port. But I've seen many an anguement for wrapped. Even seen pillows used.
I'm sure someone else will come along and make some recomendation that actually helps
