I located my fastner back far enough so that with the barrel removed it can be tightened with a normal allen wrench. You can also hold the nut on the cage side with the GL block and barrel removed. I did however have a problem with tightning the allen I have to secure the upper barrel to the frount ring. I just took a normal allen wrench, and cut it down with a cutoff wheel.
Hope that helps. Here are a few closeups of what I did with mine.