The two challenges to overcome were to make the box easily removable so it was possible to get to the clamp that fits to the tow hitch, and how to mount a flat bottomed box to a curved rack (as per your Thule).
The solution was a combination of aluminium bits and pieces I had to hand.
The long cross pieces are actually curtain rails from Ikea while the box sections and fittings are from a Karitek EasyLoad roof rack system for attaching kayaks to car roof bars. The cross pieces are bottled the bottom of the box and the whole lots slides back of the rack via the slots in the fore and aft box sections.
This was my mk1 version which was a BackUp box fitted to EasyBase using the aforementioned modified Karitek slide and tilt mechanism. The whole lot was quite heavy due to the steel tubes of the EasyBase and the the sliding mechanism I added. When I discovered the Atera rack on a VW T5 forum I decided it would much more suited to my requirements.
