the thing we do at our dealership if someone MUST have a plate on (Since we never drill them, against my policy!)

Nylon twist tie through the two holes in the bracket, go around the center part of the lower grille.......

 

Here in Illinois they won't allow you to mount the plate with wire ties, safety wire, duct tape, chewing gum, etc. The plate has to be firmly mounted. Since I recently got an equipment citation for no front plate I finally figured out the solution -- drywall anchors! Buncha pictures here:

Index of /Marty/Images/CamaroFrontPlate

You can stick the drywall anchors in the pseudo-boss holes of the bumper cover, then make an aluminum backing plate for the license plate (aluminum is very easy to machine), then mount the plate with plastic thumbscrews. Piece of cake.