Just hot glue a disc magnet onto the back of the topper and stick it to the back of your car near the license plate-like where you see everyone with the ribbon magnets do it.
I have a feeling we'll be seeing "flattened" "antenna toppers" with magnets on the back for the metal parts of cars soon enough...
In Georgia it's illegal to hang stuff from your rearview mirror as it interferes with your line of sight. My coworker was stopped for speeding, and got an extra $50 tacked onto a $118 speeding ticket for having crap hanging from her rearview mirror (beads, in her case).