Do you need that much capacity? The reason I ask is because, unless you have a center mounted exhaust, you would get by much cheaper with a hitch ($140 from etrailer) and a Harbor Freight ATV cargo carrier ($60). You would need an adapter since the hitch is available only in 1.5" and the cargo carrier is 2". The weight capacity is also only 300 pounds.
Not only is it cheaper but the aerodynamics would be much better. Having something like that on the roof will kill your mileage. On a road trip, that will add up. You can place your items in waterproof bags/garbage bags and in totes on the cargo rack. That's what I do for road trips.