I have searched my computer and the web and I can't find one. I know I had downloaded a free pattern at one time but I don't know where it has gone to! Sorry, I'll keep looking!
Here is one that is $3.49 if you can't find a freebie
http://www.e-patternscentral.com/det...755&cat_id=415
Here is a link where she talks about making her own "fabric" by sewing the selvages to muslin and then cutting it out following whatever backpack pattern you might have.
http://www.quilterblogs.com/tag/selvage%20quilt/