I don't know if there is a pattern out there for one or not, but here is a link to one you can buy:
http://www.timbergyms.com/product/Au...eping-Bag/1638
and here is a link to a homemade sleeping bag tutorial:
http://www.stitchedincolor.com/2012/...eping-bag.html
Maybe you could just make your quilt top with the alligator design like the first link has, but use the steps in the 2nd to actually put it together?