Most owners (like me) have replaced the leatherette bag with a hard-shell storage compartment and fitted it with speakers.
Porsche Boxster Rear Shelf Storage 986 551 223 | eBay
I did see pix a long time ago on the web somewhere where a guy who was too cheap to buy the box above bought the speaker retrofit kit off eBay, cut a piece of plywood and mounted the speakers, then cut into the bag and fixed it to the wood face somehow. Looked like crap though. This is much nicer and probably sounds a lot better.
Makes all the difference in the world. I'd spend $200 on this speaker box all over again if I had to. The rear speaker fill-in makes the audio system actually enjoyable top up or top down.