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No idea - its not something I've done I'm afraid. From your first picture, it looks like the metal collar that locks the o ring in plage has some recessed lugs to enable it to be screwed out and removed via the opening that you would otherwise place the CO2 cartridge in?

Speculatively, might this be them? https://www.ebay.co.uk/itm/203917328232?chn=ps&_ul=GB&mkevt=1&mkcid=28&google_free_listing_action=view_item

A bit shiny perhaps, but I quite like the aged distressed look. As long as its not overdone. Blackening large sections with heat can be a bit rubbish.

This option seems to match your description. Super cheap, come in 1m long lengths, just cut to size with a pair of rounded tip safety scissors. If you run it at the full 1m, I'm sure you will achieve whisper quiet outcomes.