Thanks for all your answers. I found exactly two different real HARD shell cases for ipad, one of which is the one proposed by booxtor - and I forgot the name of the other one. Both were too expensive in my eyes for what they can do. In essence, they are just aluminum cases (like those you use for cookies or tools) stuffed with some sort of foam.
I decided to try and build one myself. I already bought a really sturdy book (for 1 Euro) and emptied the contents (was about the Disney movie "High school musical"...).
For the padding, I got two cushions filled with a block of foam (2 Euros each) and cut the foam so it will fit the ereader with its case (from booxtor's shop).
Now I'm waiting for the decoration to arrive from ebay before I can go on. I want to wrap it in sticky foil in leather design (5 Euros including postage). This must be done before I go on working on the interior. I plan to cover the foam in fabric.
If someone is interested, I can post photos of the progress once I'm done.
This self-made hardcase will look like a big book, be quite cheap, water-repellant on the outside and sturdy enough to survive in a travel bag. I believe it will be too big for the usual backpack - but that's not such a problem for me.