I'd buy a food-safe silicone baking sheet and just cut a ring out of that. The specifications for moka pot rings are almost nonexistent, and I honestly don't think it's enough to just list "for 6-cup pots" because I don't trust that all pot designs are the same.

Found this one that says it's heat resistant up to over 400 degF: https://www.amazon.com/Ewen-27-6x19-5-Silicone-Resistant-Protector/dp/B09DK4S3G3/?th=1