Would this putty work?

http://www.nelsonfirestop.com/ They are more easily bought... they are 12" X 3" and 1" thick. It's fire-stop putty.. is it basically the same as the soundproofing putty?

Unfortunately, the link you're providing points to a Web site that uses frames, so it's not totally clear what specific product to which you're referring. If you're referring to their FSP™ Firestop Putty, that should be fine. In fact, the "putty packs" we've referenced in the past are indeed also intended for firestopping -- they just happen to add pliable mass to the box and make the box air-tight, both of which are very helpful in isolation. --Keith :mrgreen: