There are slip nuts in the track of the Roof Rails. Or often called, Maggie Rack. A pair of Unistruts from Home Depot or Lowes will allow you to install just about anything, heavier and lower than a Thule or Yakima rack. Plus you can still attach a Yakima or Thule Space Case to the unistrut, as well. Even solar panels.
Any Bicycle specialty location that sells Roof Rack Systems can get you set up with one of the many rack Systems (Thule & Yakima). Just drive your TC over to one and walk out with all the supplies you need, or go Unistrut. Pretty simple and cheaper than the factory supplied systems.
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