Looking again, I realised that rivnut would actually be better as I'd screw it up afterward.
I'm hesitating between stainless steel, carbon steel and aluminium:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F181799619948
Stainless would be the strongest. But...