Here's another thought for someone with a newer vehicle that just doesn't have the desired between the bars spread you'd like for optimal loading.  If you have a hitch on the vehicle, consider a hitch mounted load bar vs. two on the roof.  Yakima has the Dry Dock, Thule -the Goal Post, Rhino Rack T-Loader.  Not the cheapest route, but could be your answer.  If you are or know a competent welded, make your own.  I have no experience with these, but the Yak looks most stable with the 'y' design.  Adding tie downs would help stablize potential side to side swaying.  I also found the MTN Gearsmith Heavy Duty Pickup Truck Bed Hitch Mounted Extension Extender Rack Ladder Canoe Boat for $89 on line, could work, but looks heavier, less adjustable.  Worth thinking about - FWIW.

pd