I've always made mainly-cabbage ferments and always used a plain old lid of whatever container we had, and it always came out great. These are kimchi or northern-Indian oiled pickles, so they are salted, mainly cabbage with less than half replaced by other vegetables, and packed in as airtight as we could. I think maybe airlocks become more helpful if you can't use
enough salt, or are doing other vegetables, especially fruits that have more sugar than cabbage, or maybe other situations that are off the basics. We've made about 60 to 100 liters of these ferments every autumn since 2011 in less than sanitary conditions, and they've always come out great.