I have found that the "travel backpacks" can work, REI carries one and so does EMS. This one stuffs into its lid, was $19 on sale. Here is my son carrying it (~6lbs total) with the shoulder straps adjusted all the way down to where the buckles meet the pack, used an accessory strap as a sternum strap, this was his second trip with it. For the weight and the price they can't be beat. Only drawback I could see would be serious bushwacking.
Here is the link to the one at EMS:
REI has similar item as well, that might work: