Nipples are dandy, but one overlooked and very simple, effective method is a foodgrade plastic bag (sandwich, ziplock) placed over the mouth of the jar and held on with a rubber band. The flexibility of the rubber band allows gas to escape and if paranoid about the top layer, a leaf or more of cabbage (or something else) can help press the desirable layer beneath the brine. I've found that after 10-14 days the leaves become unappealing but do not affect anything beneath. Note: I ferment in Florida, with the AC rarely below 80f unless such occurs naturally. I have learned to increase the salt and decrease the time when the temp is above 77f.
Yes for sure, I need to upgrade my system at some point. Another one I've used is a ziplock bag partly filled with water. I don't love the idea of plastic in fermentation, but it has worked in a pinch.
I wish that worked for me, I've found there are folds in the bag that seem to be good starting points for mold/infection. Unfortunately I'm not certain what in my process caused this, after a few tests I decided it wasn't for me.
other nice simple one is the silicon nipple looking covers. just screw it onto the mason jar and it self burps