I suggest you make the part yourself. If you have a nut thick enough and with the correct thread, you can carefully drill and tap it through the side for a set screw. Alternately you can use it as is, and locktite it in place. You'll have to work fast to assemble the brake, and adjust the nut before the locktite sets. This is less than ideal since readjustment isn't possible, but many people use thes brakes for years without ever moving this nut, so it shouldn't be a problem.