Vaude Silkroad is pretty good. They have a few different sizes. There was a version rebranded as Edinburgh Bicycle / Revolution.
The small size is really small, barely enough for a couple of sandwiches or a jacket. But it is quite neat on a narrow rack.
I think the large is usually more useful. It has an expanding top bit. I can fit a decent camera, or a pair of shoes, plus plenty of food, and a spare tube and pump etc. Has a couple of pockets, handy for small tools etc. Plus an open pocket on the back, which you can use for a water bottle or jacket.
Just 4 velcro straps to attach to the rack - works fine, quick enough to attach/detach - so long as your rack is not too narrow. There is also a Racktime version, not tried that.
Its not super tough material, but mine is in decent condition after 10 years or so. And not waterproof, but has a cover.