The uprights are constructed of 2 - 2 x 12 treated segments sandwiched together and bolted with 1/4 inch carriage bolts.  They are then cut to shape according to Payne-Gallwey.  The inside segment is cut 5 1/2 inch longer than the outside segment, it forms a tenon to keep the upright rigid.  There is also a pocket on the inside near the top for the impact bar.
 
 
 
 
 

A series of cuts are made across the top part of the side rail on the inside to accept the tenon end of the uprights.  These are made with a circular saw set to the depth of one 2 x 12.  The excess will be chiseled out.
 
 
 
 
 
 
 
 

The frame with the uprights and impact bar in place.  The diagonal braces are not in place yet, but the frame is nearly complete.