I've an an old Oxford tank bag with magnetic flaps that fold out and grab the tank.
It's not waterproof, but has a cover stashed in the pocket at the front. There's an expansion zipper to make it bigger. You can also unzip the bag to reveal a map case. Never used it though, so a waste of space IMHO.
The least attractive feature is it only zips down the sides, with a velcro strip to seal the bottom so I'm always paranoid something might fall out (never has though).
It came with a handle, carrying straps (rucksack style) and a security loop to attach to the steering yoke. Never used them. Just carry it like a shopping bag.
I'd post a picture but they don't make that model anymore. If you have a decent top-box they're a flaff most of the time apart from long trips for passport, documents etc..
I've had it since 2005, so it's obviously well durable.