I know a few of the forum members have seen this but I thought it may be of interest to others.
I used the largest available (at the time) GIVI top box which clips onto it's own special mounting bracket that I have fixed to my carrier with some home made clamps.

The top box clips into position and can only be removed by means of a key.

The box itself is also lockable (with the same key) and is totally waterproof. It has a capacity of 55 litres and can hold 2 full face helmets.
The box also has the facility of built-in red LED lights facing towards the rear (which I have yet to wire up) and this will provide an excellent high level stop light.
The box can also be used as a suitcase and easily removed from the car and taken with you if required.

Pleas note that I have no financial or commercial connection with GIVI
Keith