Hi
We are the makers of bak-rak and sell loads to T6 owners so you should get a 'proper' reply.
However it is worth noting that we have developed a new 'tumble mount' specifically for T6 and similar that tilts the box over the outside edge so you can still open the back door. Also our systems work happily with your box so we have many people fitting such as the Thule box onto our rack which carries 100kg versus 45kg for the Thule.
Here is a picture of our tumble mount system with a different box ( the one you show is our own tough bigger and best box).
We do two types of rack for the boxes - our basic box rak (in our photo) and the much stronger base-rak that in stainless steel G4 form will carry 120kg. This can be left on the vehicle and form a step into the back.
Happy camping
Pete
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