Originally Posted by cyccommute

Look at the Ortlieb system too. They sell all the parts here. The hooks have a locking mechanism built in that releases by pulling up on the handle. Very positive attachment and can easily be released while the rear rack is loaded with stuff.

The Ortlieb Roller bags might wouldn't be a bad design to copy either. Simple bag without any zippers, just a couple of buckles. Would be real simple to make.

You could easily stiffen the back of any pannier with a thin sheet (about 1/8") of polyethylene.