The 2x2 sits on top of the van's inner body flange, with an L bracket holding it in place.
The weight of the rack is carried by the van, not the bracket. The cuts on the upright are on a slight angle to accomodate the sloping van wall.
The top of the front bracket is secured the same way as the rear 2x4...cut flush, and held in place with an L bracket.
All brackets got a hit of primer.