Although it seems like making these plug-in attachment points is adding steps to the rack build, I feel that it will make the assembly of the rack simpler, and more accurate. Start with a 1/2 x .125" tube with one end turned down to 3/8". Hacksaw off a chunk to make a nice flat section.
Weld the tab into position.
A slip fit into a 1/2 x .058" 4130 tube. These will be brass brazed into place.