I have a very old 'portmanteau'which is just the right size for one, it fits behind the seats and still leaves room for 'Odds and Ends'. For two, I bought an 'expendable' wheelie Bag from 'Woolies' which can accommodate a lot more and yet remains out of sight when in the car and easy to handle out of the car.
Just ordered my first Luggage Rack as I need room for the dog!
