I have a set that I enjoy. There are inserts with the proper straps, etc to provide a snug fit. I bought them from a buddy when he sold his 964. They do provide extra cooling/warmth in the summer and winter. I do remove them periodically to shake them out, swap sides and let the leather seats 'breath'. They add about half inch of height to the seat.
I think he bought them from this place, although the site does not show a picture - might be worth contacting them.
http://www.autoanything.com/seat-covers/69A1658A0A0.aspx
Good luck.