Using the cutter.
see posts 5 and 7 and someone posted a .pdf further along in the thread.
Prechamber Tool
I don't know why I bought it, but I paid over $300 for the cutter. I had some intentions of getting what is needed to take the measurements and maybe at some point renting it out, but it would require at least a $300 deposit and if any parts on it got damaged, even if the replacement parts cost a lot, I have no idea where I would get replacement parts for it.
I know people don't like this sort of suggestion.
Personally, if I did not have access to the cutter, I would get a thin sheet of copper and carefully cut out a sealing washer with a pen knife and make sure the retainer nut is torqued down to spec.
The most likely source of the copper sheets or foil is large craft shops. People put the sheets over a pattern and press into the pattern and end up with an embed image.
https://basiccopper.com/thicknessguide.html
This will give you some idea of the thinness of the sheets.
https://www.youtube.com/watch?v=C1_TR9Xtmn8