Is there something I am not getting here..the original post was luggage for the rear rack..most people have written about the front luggage options..
I have a rear bag by Axiom...which fits on a Brompton rear rack with 4 velcro straps..Easy to take the bag off in a few seconds..about 15 litres in all ..It has 2 side panniers that fold away but if you have large feet you might catch these..I believe the Topeak company make one similar...
The best thing I have found is simply to put a Carradice Nelson Longflap saddlebag straight on to the rear rack held on by 2 bungies ..or 3 ..about 25 litres plus..trouble is it needs to be full...