The Book Armor cases do exactly that. They're zipped, robust and roomy cases in which you can keep a reader with its own more snug, more light weight cover. My 505 lives in mine at the moment. The 505 also has the original Sony cover on it which, at the time I bought it, wasn't strong enough to protect against cats standing on it (my current cats don't do this) so required strong protection. When in the Book Armor you could almost roll a truck over it and the reader would be undamaged (I don't recommend trying this). They were very popular on MR a few years ago, but modern ereader cases tend to be quite robust anyway (my Paperwhite now has a new Finitie cover which looks like a book and is truly solid)
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
It now protects the old 505 from being damaged if I accidentally put a book on top of it.
I'm in no way connected to them.
If you don't like the Book Armor I'm sure you'll find something suitable.
Have you considered making a case yourself? Maybe from some thick cardboard or wood (padded) or even a plastic food container of the right shape? There are some excellent youtube vids about making your own cases, I'm sure you can find one concerning a rugged case.