I recommend finding a padded, zip-up (or otherwise fully enclosed) Kindle 2 case. It should fit a covered K3. I have one of
these. You could practically drop it from a building in that thing. I carry it in a messenger bag, but it may be a bit bulky for you depending on what kind of bag you carry. It was $8 when I bought it, for some reason. There are other options. Like
this, or
this.