I have made them from 1 3/4- 2" .250 wall tube (round). With corresponding solid bar for ends. The most important part, aside from precise welding, is getting the Kingpin boss at the correct angle and parallel.
If this is a one only item, I would buy one from MAS in Minn. As you will have considerable time in building a jig or mill/welding setup and getting the correct size reamer for the kingpins. The reamer alone costs over $100.00.