What light pattern did you get? If it is more for seeing right in front of you put it on top for a better angle. If it is for seeing further out put it below. I personally think below looks better and if you ever have something extending beyond the front of the rack (ex: kayak, plywood, etc.) you won't have something blocking that.
You may end up with a loud whistle if you mount it under. They sell these fins that help reduce the noise though.
I'm also wondering if their ARC series would help a bit pushing air out to the sides (and of course getting wider light spread).
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