Here is an idea for mounting the badges.

Grind off the two tabs on the back, sand or lightly grind the surface to roughen it at the center. Lightly sand the back of the earth magnet. Mix a little of the cold weld bond to bond the magnet and wait 16 hours. You must use this type of earth magnet only as they are extreamly strong. Because the badge is concave towards the center, the magnet is will be 2-3mm away from the surface of the tank. Only the badge touches the tank. The magnet doesn't touch or scratch your tank. If you want it to be permenant then add some kind of silicone to bond it for good.