Andrea and I are "back on the case" this week. She is double checking the height we need in order to handle the longest clamps. It may be higher than I want — and tippee. I am 6' 2" so I think I want it to be no taller than that with the 4" casters.  

   Making up measurements and angles is all by "best guess" at this point. The 2 x 4 that is away from her is clamped to the work surface so that it stays perpendicular to the base. The 2 x 4 closer to her will be the front, angled pieces. It looks right so we will cut accordingly.     

   Believe it or not, the red lines are parallel to each other — courtesy of the Draw program. Anyway, Andrea is lining up a guide rail parallel to the back upright. If she can cut accurately on that line, her angled upright will be cut to join the perpendicular back legs.
   Now we do have and use the Festool plunge saw and guide rail system. It is ideal for this type of project, but if you do not have it, you can use a jig saw or standard circular saw to cut to this line.  

       She makes the cut letting the saw ride the rail. All she has to do is plunge and push it forward.

      What a nice cut we have. That acute an angle is usually a difficult one to make.  The Festool Plunge Saw made it easy for Andrea.

   We decided that we would use 3 uprights, so Andrea has used the first one to draw a line of the angle and now she cuts to that line.     

   Andrea drills a pilot hole for some lag screws. 
   I have a small parts drawer marked "Lag Bolts" and I haven't looked in it for years. But since we need several, I pulled the drawer out and found two sizes that will work perfectly. (photo quality dropped measurably when I dropped camera on concrete floor.)

      While the pilot hole helps a lot, the drill didn't allow a deep enough hole for the long lag screws. Andrea learned how tiring something as simple as driving long screws is. But, the joint is a strong one.

   (Got camera back working correctly.)