I am not an AP 9x user yet but I did get one I am hoping on using for a new build soon. How wide is your external overflow? In addition to the single AP9x arm you can also use 2 A series arms. If the box is narrow enough the arms may be able to rest on either side of the box. I would also not rule out mounting off the wall.
i have 10 ft ceilings where i want to put the tank and like to hide the cable that supplys power I may have to do some welding to the steal standIf you are not concerned with the distance from the wall you could always just attach to the back of the overflow.
As I think of this I imagine Kessil support saying that some people would do 2 360Xs pver the AP9x.
Is there a reason you are against suspending the light? I have suspended lights and really like it.