I am just starting to make a cover for my wife's Sony reader. I bought a soft rubbery "skin" for it on ebay, my plan is to make my tooled leather cover then attache the "skin" by either sewing or gluing.
Something I have used and works well is linoleum. It is easy to cut and work with. The back is usually white and smooth so easy to draw on. Go to your locale flooring store and ask there installer if they have any left over pieces or scraps.