Mark 10 rivet holes (5 on each side) on the angles and the backing plate. Drill all 10 holes on the backing plate using a #40 drill. It was difficult to achieve good alignment between the backing plate and the edges of the box. Clamp the plate on the box and drill the two opposite corner holes on the angles using as a guide the previously drilled holes in the plate. It was critical that the plate does not move while doing this. Using the two holes I clecoed the plate to the angles and drilled the rest of the holes. The alignment came out great.
