Alex,
How do you attach the threaded bolt to the tube/stem thingie? Do you "hog" out the wrench flats on the inside of the head of a standard m6 bolt and braze in a tube or something else? I've been using this method on a lathe at Steve H's shop on rack builds for his tournesol bikes. He showed me the method and works well if you have access to a lathe, though we use solid rod rather than tubing for the fork crown mount "stem." We heat the rod with the torch and bend to suit with a pair of needle nose.
I don't have a lathe at home and want to build some racks in my shop this winter.
Thanks
-dan