[Hudsoner]

Have you ever considered something like this?
Customer Image Gallery for TMS ALUM-CCR-2260A 60-Inch by 20-Inch RV Hitch Mount Cargo Luggage Carrier Generator, Aluminum
It can hold up to 500 lbs depending on the hitch weight of the vehicle. You can anchor some kind of bin to it (with U-bolts)
(Amazon.com: Plano 1819 XXL Storage Trunk (Camo): Sports & Outdoors).

The advantage of this setup is that you are not top heavy and it will hardly influence your gas mileage. You could place two bins on top of each other and it still would not do much to your gas consumption. Another method would be a samml, covered trailer (similar to a motorcycle trailer).

 

[Hudsoner]

Weight limit with the hitch platform is very limited once you have the platform weight factored in. Mileage hit on the roof box isn't much till your running 70mph+ which case add head wind and you might see 3-4mpg hit worst case. I used the hitch platform once borrowed one hated it - very limited use and weight and it really caused the back of the car to sit low.

That is why I linked to the aluminum platform. It is very light weight and still can carry 500 lbs.