I started off with a 6" piece of aluminum that happened to be about 1-1/8" in diameter. Using the fixed steady rest, I faced off one end and center drilled it. Bigger...

I then turned a small amount from the diameter. This particular operation is purely cosmetic and could be skipped. Bigger...

I then proceeded to drill out a 1/2" diameter hole in the end (I step drilled the hole using the following sizes: 1/8", 1/4", 3/8", 7/16", 1/2"). With the 6" piece, the 1/2" drill bit, and the 1/2" drill chuck, there was only 3" of space left at the end of my 24" bed. The hole was drilled about 1" deep (the tip of the drill went in 1"). With the 1/2" drill bit, the outside of the drill bit went in about 0.85". It only needs to be about 0.75" deep. Bigger...

The 1/2" hole was then bored out to around 0.562", which is the outside diameter of the spindle shaft. Bigger...

I realized that the gears included with the threading attachment have an inside diameter which matches the spindle shaft. The 127 tooth gear also has a very short shaft attached to it which has the same diameter. I used this short piece of shaft to check the fit of the bore, before removing the 3 jaw chuck (with the aluminum shaft still chucked up) to double check the fit on the real shaft. Bigger...

The extender shaft was then reversed end for end, faced, and center drilled. I decided to make the end portion of the shaft 0.680" long, and used the parting tool to reduce the diameter to 0.540". This would allow a 0.010" relief at the shoulder. Bigger...

I then removed the excess material to bring the diameter down to 0.560". Here you can see a 0.050" cut being made. Bigger...

I used one of the small gears that came with the threading attachment to check the diameter for fit, before trying the handle which came with the threading attachment (the gear was snug fit on the spindle shaft). Bigger...

The extender shaft was then mounted in the vise, and a flat was added for the setscrew in the handle to seat against. I removed about 0.015" to make the flat. Bigger...

The extender shaft was then swapped end for end in the vise, and a pilot hole for the 10-32 setscrew was drilled in the center of the shaft, 0.250" back from the edge. Bigger...

A small clearance needs to be made for the head of the screw on the spindle shaft. I used a 3/16" (0.1875") end mill, and moved the Y axis 0.011" either side of center to make the clearance around 0.210". The head of the screw is about 0.2" in diameter. I decided that having the clearance angled a bit wouldn't change the functionality and chose to make the clearance like this rather than setting the mill up as a horizontal mill. Bigger...

This is the finished screw clearance, along with the installed 10-32 setscrew. Bigger...

The finished extension. Bigger...

The extension mounted on the spindle shaft. Bigger...