You could also consider installing something like the Yakima rails or landing pads.
Click on the "Instructions" link at the bottom of above page to see how these install.
I have these on my Honda Insight and they allow about 41" spread b/w the racks (vs. about 25" or so with door-mounted).
The "catch" (other than having to drill through the roof, which is actually quite easy if done right), is that you have to know *where* it is safe to drill. I got the shop repair manual for my car so I sort of knew where the wires, roof frame, airbags etc. are, so I was not drilling blind. Another issue is that if you drill in just the sheet metal, then your load capacity will not be more than about 60-100lb (your roof will begin to bend visibly with more than 80lb). So you could do like someone else here did on their Mini Cooper - drill into the roof support beams and use different hardware to attach the landing pads. This way you will have much higher loading capacity.
Lastly, if the rear of the car is too slanted down relative to the front, you might need to compromise the spread to get the rack mostly level with the ground...
On my car I have quite a bit of curvature in the front, so the rails did not seem like they would bend well enough. So I used the landing pads instead.
One more consideration - on my car the rear of the roof line is a couple of inches narrower than the front. If you install rails they will be at an nangle relative to each other - not sure if that would be a good thing for the control towers that need to slide in them. No such issue with the landing pads - you can have each pair be parallel; my front pair is 2" or so wider than my rear but the 2 pads in each pair are parallel to each other...
Deck Rigging Gear
Heel and Pegpads™
Touring Kayak Paddles
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