I just got the Z out of hibernation and decided to address this. I was able to find black adhesive back felt sheets from the hobby/craft store. They were very inexpensive 12" x 12" sheets (~$1.50 per sheet). I bought 4 but only ended up needing 2.
I started with a good cleaning of the back panel trim using just hot water and mild dish soap. I wanted to be sure all the oils/dirt were removed before trying the adhesive. Next I slid the seat back and outlined it with painters tape, leaving about 1/2 inch past the seat edge all around. That gave me a good idea for the shape. Then I just slid the seat forward and carefully trimmed the felt to match the shape, removed the backing and applied. I was able to do it in 2 sections with the seam at the middle. The felt stuck nice except it was a little loose around the roll hoop trim (sharp radius there). I slid the seat back to apply pressure and let it sit overnight. With the seat back, you can barely see the felt at all.
I've driven it a couple times now, and although the squeak is not 100% removed, it made a big improvement. I think the leather is just so squeaky, even against felt is will squeak sometimes. I'm going to drive it a bit more, and if it is still bothersome, I may apply felt to the seat as well, so it is felt on felt contact. I definitely think this is worth the $3 and 1 hour it took to fix!
