There's the Wexler Flex One. It's tough due to its flexible display which doesn't crack. The form factor takes some getting used to, though. I don't know if any other readers use this display yet.
Another reasonably tough model is the PB360. It comes with a lid so it's sturdy even without a case. But as all other eink devices, the screen is just glass. So it will crack under stress; one user managed this recently by putting it in his ass pocket and bending down
pants can exerct quite a lot of force on such a device
Tough cases can only do so much too. Might help with drops and regular backpack bumps, but torsion is a bitch and if your backpack weighs as much as you do...
No matter which device you get, you'll just have to devise a way to treat it very carefully.