Hi Dave,
I was missing the steel plate (idiot). Installed as you suggested and went out today. First 200km of hiway at 120-130kph not a twitch. Worked a treat and provided plenty protection. Then 20-25km of rough gravel tracks with plenty washboard type surface, again stayed in place no problems. However on the return journey, a blast of 180kph found my handy work wanting and I managed to remove her before she became a deadly flying object. Back to the drawing board but must get it right as the thing works when you can keep it in place. One question, did you use locktight or such on the holding screws?
Regards