Sixteen self-tapping screws around the perimeter ought to do it. Add an extra in the center if you feel it's still moving around a bit.
Barring that, some waterproof 3m VHB tape should do the trick. Cleanliness is next to godliness when using adhesives. Wash, rinse, degrease, wash, rinse, degrease and when you think it's clean, degrease again after you degrease one more time. Denatured alcohol is your best friend. Use it, love it, but don't drink it.