I got a roll of the non-slip rubber stuff from the green shed.. cost maybe $10 ? Cut it just wider than the metal and placed it 2 or three layers thick on both sides of the cross-bar.. tighten down.. hasn't moved since.
Be careful though.. the rubber do deteriorate over time - for example.. don't put it under the roofrack pillars, it leaves horrible marks on your paint that's a pain to get off.. don't ask me how I know..