Meatball from Hell on Wheels made a suede seat cover for one of his builds and is now offering them to everyone. The seat he is using appears to be off a Thruxton but would easily fit on the Bonneville or Scrambler as well.
It's an exact copy of the factory pattern and uses the stock foam. They can
make you one in most colors for $160 plus shipping. All you do is send them your seat (and money) and they'll do the rest.