B&Q foam mounting tape its water prove and easily strong enough. I had my plate held on with just one strip through heavy rain, winter, high speeds and all that combined, the thing thing was still hard to pull off after a year.
i have mine stuck on with black silicone.wont move for big grade.just hold it for ten mins or so then stick tape to secure for an hour or 2 den come out and take tape off and ul be doin well to move it