I am not aware of anyone that makes gas tanks specifically for the Panther. However, there's a guy that has fabricated tanks for the worksman bicycle. If you google "Sportsman Flyer" and shoot Pat an e-mail, he maybe willing to build you one. He is very reasonable, considering the fact this would be a one-off.
Another option is the small keg style tank that attached to the seat post from www,piratecycles1.com.
I also know one guy who fabricated one from wood and sealed the inside with POR-15 (tank sealer); and believe it or not has been using it without problems for over a year.
Best of luck in your quest for the right gas tank to compliment a beautiful back.