They are what the proffesionals use. They will do no damage to the paint and I have hauled 1000s of cars with not one scratch. Many cars were 50K+ cars. They are designed for that reason. They are made for the magnetic tow lights that are attached to the towed vehicles trunk lid or roof. Those are on eBay for about 7 bucks but the seller wants like 13 bucks shipping. A tow supply company should have them. Here is the ebay link and these are too high priced on the shipping.
http://www.ebay.com/itm/Magnetic-to...her_Vehicle_Parts&hash=item2a1f6f64ed&vxp=mtr
You might check this wrecker supply and see if they stock them.
http://www.awdirect.com/
Rubber Magnet Covers
Fit all 3 to 3 1/4 diameter round magnets. Imported.
http://www.awdirect.com/rubber-magnet-covers-mb100/tow-lights/
They also have the Bruise Tape
Use this self-adhesive plastic film to protect your customer's car from the minor "bruises" that can be caused by magnetic tow lights, door tools, tow straps and more. 5 mil thick, this clear tape comes in a convenient 200 foot roll and is perforated every six inches. Simply tear away at the length you need and stick it to any metal or glass surface. 6"W x 200'L roll. Made in USA.
http://www.awdirect.com/bruise-tape-bt260/tow-lights/
Another way is to get one of these and used the magnet or rubber of it with your antenna.