Making the drag braces was fairly simple. First I calculated the angle to be about 45 degrees.
I nailed blocks into the workbench as directed in the manual so I can move the rear spar sideways to show installing the drag braces. Once installed I push the rear spar against the block to bring the wing square.
About five tries removing a tiny bit of length each time I had a perfectly fitting drag brace. Repeat for each one.
Each drag brace is supported by some gusset blocks made from RS-11. With the miter saw I had these cut in a few minutes.