There are several manufacturers of roof racks for the Dodge Promaster/Fiat Ducato. The Fiamma brand that Winnebago uses is just one. Do a google search. The racks attach to welded on tabs on the roof so you will not have to drill and the mount is very secure. Very easy to install. I think a Dodge/Ram dealer might even have something to look at. Try etrailer.com or autoanything, also. There's a bunch out there. Make sure you search both "Dodge" and "Ram" as the auto make.
After that, just pick the box you want but a warning. It's 9.5' to the roof. You're not going to use that box without a ladder, which you now get to bring along everywhere. Your mileage will also take a hit. And don't think about getting up on the roof. It will dent easily.
Look into a rear cargo carrier as an alternative like a stowaway. They're not cheap but neither is a rooftop carrier after you add up all the parts you'll need, (rack, box, ladder). The down side is that they add to your overall length.
"Il Travato Rosso"
2015 Travato 59g