Diddly, I put a big washer on each side of the rubber grommet to "squish" it tighter, haven't had any trouble. I'm using incandescent bulbs going down a rough dirt road where I live.
I know the post is late, but you can omit that bracket if you want. Enjoy the ride. P.S. don't be looking back as your going down the road to check how cool they look...it'd tempting.

