A couple of tips: The strongest support on a car roof is at the edges, where the roof rolls down to the sides. If you're concerned about structural strength (as you should be), avoid loading the flat portions of the roof; padded curved metal feet that fit the outer portions of the roof will do an excellent job of distributing the load safely.

As for tie-downs, I've found that , if there are no sturdy tie-down points provided on your car, it pays to install some eyebolts attached to something solid (like a bumper bracket).