Buy it on amazon: http://boxvimo.com/affiliate-link/

...these covers snap over a plastic collar (included) held in place by the screws (also included) that replace the OEM hardware that holds your license plate to the car.

Once the screw and collar are in place, the covers snap on with moderate pressure.

I can't know yet how they will stand up to the elements over time, but they look much nicer than a plain old screw.