This might be a little more than you would want to give away but here are a couple of fold up Maxpedition backpacks,

http://www.maxpedition.com/store/pc/ROLLYPOLY-Folding-Backpack-12p506.htm

http://www.maxpedition.com/store/pc/ROLLYPOLY-EXTREME-Backpack-12p474.htm

Have not used either one of these so I can't comment on the usefulness of the backpacks.