I think it is a measuring thing since cards are different sizes. If you bring up Carol Duval and search for that it will probably pop up. I know she has shown how to do it over the years.

Here's a site that has a box made of Christmas cards (or, of course, you could use other seasonal cards... Easter, St. Patrick's Day, Hallowe'en, etc.).

There are other cute ways to recycle cards there, too.

I know I've seen a site somewhere that has a fancier box, but can't locate it right now. Maybe someone else will find it for you...

Here is a link that might be useful: RECYCLED GREETING CARDS

that was such a nice site. I'm saving it.

These aren't free, but they're neat to look at anyway:

http://www.inkastamps.com/boxes.html