as a kinda out the box solution:
I have been looking at trunki yes the kids travel cases
these things
http://www.google.co.uk/products/cat...ed=0CGwQ8gIwAQ#
I figure if they are child proof then they will probably be more than up to life on the back of a bike
I found them for �19 so you can have your two panniers and a top box for the grand total of �57
I rekon you could mount them decently using a proper rack
and the best thing they do all sorts of designs I kinda like the orange zebra style could paint the beast to match !