The Thule SmartClamp can be fitted to some Ducato, Relay and Boxer vans to give you a rail that can be used for various applications, including fitting a Droprack. You can use a Droprack on a Thule SmartClamp if you have this railing channel system fitted to your van.
The Dropracks TVE1 (Tall Van Edition) would be our recommendation for these vans.
You'll need the 751 footpack and 3075 adapter kit to fit a Droprack to the channel on the top face of the SmartClamp system.