Lancer, those are also Kuryakyn arm rests.
I had some 1/4" flat aluminum laying around and just made a wide "H" that would fit on the bolts between the frame and the top box mount.
This gave me a place to bolt the arm rests to the sides of the "H". Eventually I'll be adding some more lights to the rear parts of the "H".
Everyone that has ridden on the back says how secure it makes them feel.
Hope that helps....Sonny