I fooled around with a non-rigid trunk for a while:
http://ecomodder.com/forum/showthrea...ast-18717.html
Here's a bag with taper in it already. It's got a lot of buckles on one side but can be flipped over on a motorcycle. Nice thing about this solution (if it is one) is that it probably is not going to lock you into the bike if you crash, and it is easy to jam it forward against your back. I think you might be able to put some inflatable device inside and pump it up fairly hard, but if you want to load it up with cargo the inflatable can be removed or deflated a bit to make room - then after loading, pump it up again.
Amazon.com : OverBoard Waterproof Kayak Deck Bag, Yellow, 20-Liter : Boating Dry Bags : Sports & Outdoors
For higher up on your back, maybe one of those motorcycle aero backpacks I have seen?