My old drawing .. have to check the dimensions for accuracy. This drawing is provisional only ... needs to be verified
http://www.oocities.org/vk3cz/CS8page.html
The best method is to buy 3/4 tube and solder the lower channel section onto the tube , brass would be a good material , easy to solder . Looking at the original pics, the lower section, it appears to be a one piece steel shape , which is nigh on impossible to replicate unless you have a big press.