This is the view inside of the ceiling with the new wire pulled in through the hole in the laminated beam.

The completed installation of the Lutron Diva Dimmers

The light fixture hook and bracket are attached to the deep octagon electrical box in the ceiling.
The two gang plastic old work electrical box is mounted to the wall by tightening the screw in the upper right corner and the screw in the lower left corner. The wings will be pulled from behind against the wall and create a tight wedge. In addition a #8 x 1" sheet metal screw was driven through the box into the adjacent wall stud for additional bracing.