What I did on my dirtbike, was get a piece of lexan. Cut it to match your plate in size, then use some self tapping license plate screws to create the holes that you'll use to attach the plate. But before attaching the plate, drill a couple small holes in the center a couple inches apart right below your brake light there and pop rivet the piece of lexan to the plastic piece. Then attach your plate, and if you want a clean finish, you can zip the excess screw on the backside off with a cutoff wheel. It'll look clean and it'll stay attached.