A long time ago, I wrote a post about making a portfolio box. Ever since, I have had a steady stream of hits from google on that topic. I don’t have the time to write a tutorial myself, and I not an expert either, even if I have made a few since that last post, but I can at least provide a couple of links.
http://lilbookbinder.wordpress.com/bookbinding-portfolio/boxes-and-portfolios/
http://bibliocat.wordpress.com/2007/04/21/making-a-clamshell-box/
Enjoy
In The Pocket 