I have a tonneau cover which was an Exclusive option for my '99 986, now departed. It installs by clipping over the metal panel that comes down over the rear of the softtop, and on little metal studs which are installed across the dash and the doors. There is a zipper down the middle so you can sit in the driver's seat while it's on.

The only reason I got it was that the car was already packed and in transit. It listed for, and I paid about $1,000. It's never been used, except once I put it on in the garage to see what it looked like. I think Porsche was trying to recreate the James Dean look, but honestly, the power softtop is much easier to use than this thing. It would enable you to be more comfortable in cool weather by keeping your entire lower body below the cover and well heated. Your ears might freeze, however.

Anyway, if anyone would like it, it's available. Make an offer.