Porsche Rear speakers

[Perfectlap]

I think it has to be a certain kind of filler, one that has sound dampening qualities. Perhaps something like Dynamat or polyfil wadding. I had some put into the door panels of a previous car (behind the speaker) and it made a tremendous difference. I think the concept would be the same for the rear speakers. There's really no sound dampening behind the seats, just the panel and the soft top.