my girlfriend has made tons of these bags:
http://noodle-head.com/2011/06/summe...pattern-2.html
I've been the lucky recipient of two of them and I love them. She's got them down to taking about an hour a piece to put them together. Noodlehead uses canvas rather than interfacing for sturdiness and a good hint for cost savings is that the canvas is available really cheap by going to the hardware store and purchasing lighter weight painting canvas drop cloths.........