I too use an 80 litre Overboard bag. I actually bought it as a dry bag to keep my wet gear from kayaking in and prevent the amp escaping outwards into my tin top.
Then my ever clever wife suggested if it keeps water IN it should also keep water OUT. This has proved to be the case and it is now mounted on the luggage rack whenever we do a longer tour.
For a weekend break we use 2 Hills Alive cases , yes they are expensive and simple squashy bags can fulfil the same function, BUT they do maximise the volume available and are very easy to stow away. If it fits in the case , it fits in the car.
Remember that a heavy load mounted on the luggage rack is not good for the balance of the car. It can contribute to tail-happy behaviour.
