The general problem with this is that the pipe that is easily available to me does not quite fit my turning tool shafts. It is a bit too small for a 5/8" and a bit too large for 1/2". I drilled it out for the 5/8" and shimmed the 1/2" with copper pipe, grinding it away until I had a fit.
Incidentally, for the limited expansion needed to make the 5/8" shaft fit, an ordinary 5/8" spade bit will work in the galvanized pipe. Wear hearing protection and take your time but it will work. As always, safety considerations are your own.
Insert the set screws and assemble tightly the extension to an elbow to the 2" nipple to an elbow to the 4" nipple. You do not want these to rotate under use so tighten hard. Some thin CA trickled into the joints can not hurt.