If it’s a hard case, another option (a bit DIY) is to get mounting hardware from Ortlieb or some other pannier company and mount it to the portfolio. Then it could easily attach to a rear rack. You could set up the mounting brackets however it makes sense based on the riding position/size/etc.
http://www.bikecommuters.com/2008/01/08/pelican-case-mod/ has a video of how one person did that.
