I attempted to salvage some offcuts and test pieces to make tripod mounts.

This first piece was too small to make the part in one setup. I'm trying to do it in two without making that too obvious.

Removing 0.2mm has it at the correct diameter.

Now I need to grip the finished diameter. This is a strip of kitchen-grade aluminium foil folded to give two layers between the jaws and the part.

It's close enough. The jaws have left very slight marks, so on the next part I used four layers of foil which worked. The circumferential line is visible but can't be felt, so I guess it's down in the 10μm range. It also varies around the circumference, and is non-existent on the other side.

I'd never accept this for a visible part, but this one is always hidden inside the assembly casing.

This is another piece of aluminium I had been using for test cuts.

Drilling the centre hole, which will be threaded 1/4"-20UNC.

Parting off. This still goes poorly but only gets really bad at the centre. I was hoping that with the hole drilled I would avoid the worst of it, and I did.