Linky shows a roll bag like Mr. UKI suggested. I had another thought. Why not replace your current rack with a Ventura* pack system? You get a 'normal' rack for use like your current one, plus a big pack and extended frame to support it for when you need it? See
http://www.ventura-bike.co.uk/
Pete and Lissa have one on the Duc IIRC.
*I've seen the prices, not cheap!