Why not just buy an overbag? Not as light but much cheaper than buying a new bag.
I am thinking of getting one for my next trip
Either PHD or Wiggys, both have similar price and performance and it will be a postage cost decision
http://www.phdesigns.co.uk/product_info ... ucts_id=60http://wiggys.com/moreinfo.cfm?Product_ID=67