I wouldn't worry about scratches, as long as you don't use them for tow truck lights. And I would not use anything between the magnet and body that would decrease the holding power of the magnets, even slightly.
Also, if it's for just one or two rods, an old trick is to lean them on the windshield and use the wiper blade laid over the rod handle to hold them down. It works surprisingly well.
And I would not use anything between the magnet and body that would decrease the holding power of the magnets, even slightly.
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