If you dont want holes you could use outdoor double sided adhesive. Scotch 3m makes up to 100lb test which would almost require you to yank your bumper off to remove. Me myself, i just said screw it. Had the dealership drill the holes then went home and widened them and got a locking bolt set that requires a special key and the hood to be popped to remove my plates. For 5 bucks each plate it was worth it.
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View my fuel log 2014 Mirage ES 1.2 manual: 36.2 mpg (US) ... 15.4 km/L ... 6.5 L/100 km ... 43.5 mpg (Imp)