I did look at the reference you posted, but didnt see it until after I was finished yesterday.
Actually, mine is prettyclose, has an upper and lower box, the 2x6s were cut 45 degrees and joined. I did use 4 x 4's but they were notched upper and lower and fit into the upper and lower box. (3/8's carriage bolts were run through the 2x6s into the 4x4's. Also screws were used in spots for additional reinforcement as well.
I also used 2x4s to reinforce the upper and lower boxes at 3 points on each side and one point across the front (my door hinges will attach there. I did not "gusset from front to rear in the center, but I think thats a good idea and I will cut one 2x4 down today and do that.