Rubbermaids? Yes, Rubbermaid containers.
Check out The Tale of Two Rubbermaids, which is where I got this idea.
Mary (of Owlhaven fame) used Rubbermaid containers to transport her donations. Rather than buying more luggage (duffle bags of this size start around $25), she packed up these containers, secured them with zip-ties, and used them as part of her checked luggage allowance. (Note- she strongly recommends the "roughneck" containers as other brands have not held up so well...)
(Bought my zip-ties at Home Depot today:)