This is a prop adapter for an RC helicopter and the dimensions are pretty darn close to what I've seen. Plus it comes with grub screws so the only thing I needed to do was shorten the shaft, and drill out the centre hole to fit the 3.1mm brass rod I am using as a lever.
Here's the details of the part. And if the seller I got it from was here (eBay link).
I went for an M8 for shaft Ø2.3mm, even though they do a 3.2mm because I wasn't sure if that would affect the overall diameter of the piece and it worked out really close to the dimensions in the plans (except the height of the grub screw section, but I have seen that vary in other builds).
It cost me under £5 shipped, and shipping was really fast for me in the UK. Outside of the UK and you'd probably be better buying one from one of the US members that make them from scratch.
Started by clamping the piece down (with the nut attached to the thread to keep it level and parallel to the table. I started carefully with a dremel to give me a groove, but then finished 95% of the cutting with a hacksaw. Just take it slow not to slip. Then finished the last cut back with the dremel. Cut the shaft slightly longer than you want it to be to accound for blade widths and tidy-up with files.
Then just using files, tidy the cross section and get it down to size. Slow and smooth is the key on a long file.
All that's left is to drill the centre hole to size. I started with a pin vice with a titanium bit, and then switched to the cordless drill, but still not too fast.
That's it, pretty easy and I'm really pleased with the results. This will replace my resin part and was the last part I wanted to use real metal for on the thrower.