You could look at securing a sub-structure to the inside top to use as a base for your outer structure. Then on the outside, configure a carrier tray that covers the CBC and swings down to cover the cab, ie, take a look at Nissan Xterra basket/luggage carrier. With the design of the Xterra with theater rear seating (sitting higher than the front), the cab has a slight rise to it. Your cab and the CBC have a slight rise to it.
The design of the trays will have to be configured to how much weight you are placing up there and how high the overall height will be for wind pressure (creating push/pull on the structure miounts).
