Wandering through Ikea this morning and found this blue poly zippered duffel bag for just $2.99. 6.6oz (185g) and 28" x 13.5" x 10".
It is designed to hang on one of their rolling carts, so it has some extra straps on the back that could be removed and save more weight.
I have used the Ikea open shopping bags for gear storage and car trunk bags, but the zipper helps with more dust-proof storage. The open bags are perfect for stashing muddy gear on the way home.
I think you could get away using one of these for an airline bag for a backpack, but I would be tempted to add a couple webbing straps or even some fiber packing tape and zip-tie the handles together. It's cheap enough that you could donate or recycle it at the destination. It swallows up my GG Mariposa pack with room to spare.
http://www.ikea.com/us/en/catalog/products/90149148/?query=901.491.48
Product dimensions
Depth: 13 ¾ "
Height: 11 ¾ "
Max. load: 55 lb
Volume: 20 gallon
Depth: 35 cm
Height: 30 cm
Max. load: 25 kg
Volume: 76 l
