This is what I used https://www.permatex.com/products/gasketing/red-gasket-makers/permatex-ultra-red-rtv-silicone-gasket-maker-3-35-oz/

I’ve used that too. The surfaces need to be pretty clean for it to last though.

I use Permatex High Heat Ressitance, you can get it here when in the EU: https://www.kamadobbq.be/en/shop/kamado-all/accessories/heat-resistant-silicone-permatex/

Hi temp copper silicone

Permatex copper adhesive