If you have a welder, or a friend who does, weld a piece of flat stock on the end of some round stock and there you go!
Best to have a good tight fit up.
I have machinery so I turned the round stock to get a better fit on the smaller diameter actions and milled the flat stock for a good fit up on the lug abutments. I even case hardened the hot rolled flat stock. I doubt that's required. I did to break up a can of Kasenit I had collecting dust on the shelf.
I put tee handles on them to make sure I could get the required torque easily by hand. However, I'm thinking of cutting them off and welding on a socket to use a wrench. Doesn't really matter if you know how tight the barrel needs to be put on.
PM me if you want more specifics.