Most Doblos have built-in mounting plates with threaded holes (M6 if I remember rightly) for fitting roof bars/racks. If by any chance you are the Chunkolini from the MIG welding forum, you’ll have no problem making some roof bars to suit.
Inside width of my Doblo is a fraction less than 4ft, so a board might need to be tilted sideways. The board would probably have to hang out the back a couple of feet. If the vehicle has a top-hinged tailgate, the lights and indicators will probably still be visible, though you would still need to tie on a number plate. Models like mine, with the side-hinged double doors, would make it difficult for following traffic to see the lights.