What do I need to know about a roof cargo box?

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Mazda CX-9 Grand Touring 2016
It's never too early to watch craigslist for a roof cargo box for skis for next winter.

2016 CX-9. I have the rails, so I'll get the Mazda crossbars. (Anyone want to sell me crossbars?) Mazda 0000-8L-N11 crossbars cost about $175 from the mail order outlets, they have a 200# weight rating, and are wide enough for my needs. I don't see a need for $550 Thule or Yakima bars.

I'm looking at the usual suspects, Yakima and Thule. Anyone have a preference among the two? Why? I haven't owned a cargo box.

I think I want a low one for better aerodynamics and to fit into some of the places I'll park. I probably need a box about 36" wide or so for 5 or 6 pairs of skis. Probably need one nearly 7' long.

What do I need to know?
 
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2018 Mazda CX9 GT Crystal Blue
Just get one that opens from the side. Rear entry on the CX9 would be very difficult with the lack of bumper to stand on and the long reach over the hatch.
 
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Mazda CX-9 Grand Touring 2016
Thanks. I found just what I was lusting after, a Thule Motion XT Alpine, at a good price on eBay. I'll pick it up this weekend.