The $1 Kindle 4 Cover
amazopia.com
amazopia.com
Tear out half the sheets, and the notebook holds the tablet perfectly, plus you still have a note-pad you can use. The elastic at the bottom keeps it closed. The pocket tore at the bottom, but a bit of duct tape solved that.
Classier looking than an envelope, but super cheap and functional.
http://www.amazon.com/CE-Compass-Cover-Kindle-Black/dp/B004A...