Are you sure that is not an 18mm plate? I can measure a 9mm plate at work tomorrow and see exactly how tall it is. Also how far is the edge of your bore from the edge of your door? 4 to 5 mm is standard.
Mike, I don't think you have to worry about the heat coming off of those T8 lights, I don't think they get very hot. I have cardboard boxes next to some lights at work and they don't get hot.