I have a Thule Summit box that we have had for 7 years now. It'll work with factory cross bars, Thule square bars and Yakima round bars. We use our with our Yakima cross bars. It's 7 1/2 feet long, 2 feet wide and about a foot deep. We probably paid $400 for it at the time. Not too bad considering how long it should last. Heck our Yakima Steelhead bike mounts are 12 years old now and doing fine.
The Thule's and Yakima Rocket Boxes are very well received and I used to sell quite a few of each when I was working at a bike shop when I was younger.