If it's the metal roof, then it shouldn't be a problem.
If it's the glass roof, then you need to be careful. Strapping anything down that touches the glass could prove problematic when buffeting occurs. I'd suggest making sure the item is only touching along the roof rails and there is plenty of clearance in the middle so no pressure is put on the glass. Putting some strips of wood (such as 2x2's) along the rails can help create space, if needed.
Alternatively, buy the crossbars -- they look good.
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