I got myself a Givi TPH02 Tanklock tank bag. I had a Joe Rocket magnetic one but sold it as it scratched the paint. This one has a mounting ring that bolts to the gas cap ring, and the bag clicks straight onto it in a second. The bag never touches the tank. The upper of the bag is a textured plastic, with a window that allows an i-Phone or similar to be displayed. There's a removable, soft-lined tray for sunglasses and keys.
It's only 4 litres, but it's a useful size for wallets, glasses etc and it looks good on the bike. Comes with a waterproof cover too, but looks like it would take a shower without too much drama.