This is the top of a child's Halloween prop I bought at Toys 'R' Us attached to a light (75-lb pull) salvage magnet with some epoxy and a single screw.
The magnet is very strong and holds the ornament in place securely under normal city driving conditions, although I have not tested it at highway speed. It is easily removed by an adult, and can be securely positioned on any smooth ferrous metal surface.
What can I say? Skulls are cool.