I have a Hepco and Becker "Junior" 45 litre box for my adventure with a dedicated metal mounting bracket. The Bracket was £50 and the box just over £100 from Motobins. I've also added a luggage frame and back rest to the box which cost an additonal £50. The kit is made in Germany and is very rugged. In fact far more rugged than the stuff from Italy and with the rack on top suits the look of the GS or Adventure.
The main initial reason I bought Hepco & Becker was that they were the only company offering a dedicated mounting plate for the Adventure. It fits very well, is made of metal and allows the box to be mounted easily and removed easily. I've also changed the badge to a BMW one and it looks great and offers enough capacity to hold 2 Arai full face helmets and gloves for 2 people.
Simon