Tonight I only had a short work session. I deburred all the holes on the backing plate then fitted it. I ground just a hair off the left side of the backing plate (see photo 2) for just a bit more clearance from the tube. Next I attached the backing plate to the tube in order to perform the next stage of locating and drilling the inboard attach angle to the tube and backing plate.
Took a few minutes to locate the inboard attach angle to the support tube and battery tray and cleco clamp them together to get the location marked.