I just happened to look at my Reader versus a DVD clamshell and noticed they were the same size other then the Reader being 3/8" shorter. Having mailed DVD's in their clamshells before it reminded me of the rather ridgid cardboard or corrugated plastic shippers. One could use a mailer and glue contact paper or cloth/etc on the outside or if one were more handy use it as an inner liner for a sewn work with a clasp and strap perhaps. The following link is for reference, you'd have to buy several whereas I know different shippers, office supply stores and the US Postal Service sells them individual for a couple bucks.
http://www.meritline.com/dvd-mailers.html
-MJ