I'd recommend the tuff-luv case. It does add bulk, but also protects the reader well. Even in the case my reader takes up less space than the average book I read, and I'm confident it's protected well enough for the things I do with it.
However, I haven't used any other cases/covers except the one that came with the reader so you might want to hear what other people have to say. There is a discussion of the tuff-luv
here which will give more detail about the case, and photos.
If you have any specific questions about the tuff-luv then ask and I'll try to answer them!