I hope to add to this page in the future as others share their deck bag designs. So if you've made a bag for YOUR Deck of 1000 Spreads, let me know!
Pearl Annie from Australia made this FABULOUS deck to hold BOTH her Deck of 1000 Spreads and her Deck of Lenormand Houses. Basically, she's cut a long rectangle out of sturdy felt (darker blue), then curved one end to make the curved top. Then she's just taken two small rectangles (lighter blue) to make the sides. Add a button, make a button hole and viola! Here's Annie's video explaining it and showing more of the bag.
Seriously. This is a bag anyone can make!
Annie's finished all the edges with a blanket stitch, but the nice thing about felt is that it doesn't fray. You could just cut it out, sew the sides on and you're done. If a buttonhole intimidates you, use a pin...or better yet, buy a sparkly fancy one from a flea market. Having a hard time finding good felt? Get an old 100% wool sweater and wash it in hot water and dry it in the dryer. It should shrink up to at least a third of its original size. And there you go...you've got a thick wool felt that you can cut and it won't fray and it will be your favorite bag for forever.