I'm using a British military surplus Mk II gas mask bag, very much like the one shown in the link below. The main difference between mine and the one in the link is that I also fastened the bottom of the bag to the seatpost to keep it from swinging. The bag works great, looks good on a vintage bike, and only cost me about ten bucks on eBay.

Make It: The $10 Bicycle Saddlebag | WIRED