I agonized over which tank bag to get, and I'm very happy with this magnetic Cortech Super 18. It fits the FZ6 like a glove. Cost about $115 at my local dealer.
Not too big, not too small
Looks like it was made for the FZ6
Very high quality, easy operating zippers
Sticks like glue to the FZ6 tank, VERY stable and secure
Small-ish main opening
Small-ish map pocket