Rhinowalk 40-60L tail bag.
$120 from Amazon.
It seems nice. Bunch of straps and clips, fold flat for storage, expands to 60L. It's rigid, there's reinforcment in the panels. I wouldn't stand on it, but it'll stay up. The photo shows what it looks like empty.
It's also heavy, heavier than you'd think. This is not just a cordura bag. It has some heft to it as is empty. Includes shoulder strap, which you will likely need.
Supposed to be water proof, but it comes with a rain cover, so I assume they mean waterproof with a rain cover.
Fixed to the bike with straps and those common fork clips. A nice feature is the clips have a hinged clamp in them, so once you set them, you clamp them down do they shouldn't go anywhere. Clips are a little stiff, I probably wouldn't want to take it off with cold fingers. There's also velcro tie wraps to secure the extra strapping.
Well thought out, just got it for a short 2 night jaunt this weekend. Way more than enough for that.