The middle section of my dog walk was supported by two sawhorses that were easily put together using pre-made brackets. The only problem (other than schlepping the board and the two sawhorses around for mowing and course changes) was that the dog walk was rather unstable from side to side. In the video, the end planks aren't attached, but even when they are, the instability persists.
The first step was anchoring the short pieces of 2x4 to the ends of the plank. Ed used three big bolts to secure them. While he was doing that, I used a miter saw to cut the top of each of the legs. Then I measured 49.5" along each edge and used the miter saw to cut them to the correct length. Because Ed was a tool and die maker and I'm a quilter, we couldn't help ourselves and we also put a 20° on each end of the pieces to which the legs would be attached.
This actually was helpful when it came time to attach the legs since it gave us a little more of a guide to eyeball the leg angles. Ed insisted on using two bolts on each leg.
Once all four legs were attached to the plank, we put the whole thing on the ground upside down so it would be easier to put on the two stretchers. Then we put the whole thing back up on the sawhorses for Ed to work on the center braces.
Fortunately, we had leftover truss brackets so Ed didn't have to put anymore bolts through the DW plank. Instead he used two truss brackets.
Cutting the cross braces was the most tedious part of the job. There is no easy way to determine the angles, so it was trial and error. By the second brace, I discovered that I had to cut the piece about two inches shorter than the line I scribed with the cross braces resting on the 2x4 leg stretchers. To make a more flush joint, I changed the angle by a degree or so.
We completed the job after lunch today, and then were faced with the daunting task of getting our Paul Bunyan sized sawhorse out to the field. It weighed quite a bit because I opted for green-treated lumber so I wouldn't have to paint the legs or worry about them rotting. But a green 2x4 is really, really heavy. Surprisingly, we were able to carry it the 200' feet or so to the field without injury or heart attack. However, we will be adding a set of wheels in the future.