I had a set of Blackburn racks on a Long Haul Trucker. That bike had the mid-fork mounts in the center of the fork which is common for non-Thorns, not in front of the fork which is the norm for Thorns. I have had trouble getting other racks setup with a horizontal bar on my Thorns for that reason.
I attached a photo of the setup I had, but I no longer have it. You will notice that on the Blackburn there were conical shaped depressions drilled into the diagonal brace and the sheet metal bracket that went around that brace had a corresponding point punched into it to keep the brace from sliding. It appears that your rack lacks those depressions that the Blackburn had.
Not sure if this is any help, but it gives you an idea of what it looks like on another bike.