How about a tow-bar mounted one?

Convenience wise Would be less of a pain in the cods than a box on the roof?

How come you need a roof box with all the space in a van?

Why do caravanists put bikes or boxes on the roof of the tow car and then tow an empty box on wheels behind them???? Does it actually make the setup more aerodynamic or I guess it's so they have a rack or box usable when the van is at the site???