I love mason jars and have them in several sizes. I also buy the plastic lids and have had them for many years. No rust!
Here are a couple of patterns for covers. The first two have a round bottom, so may be a little more difficult. The others have a square bottom and don't seem ill fitting.
https://mountainrosedesigns.blogspot...-jar-cozy.html
https://modafabrics.wordpress.com/20...-jar-tea-cozy/
https://blog.thermoweb.com/fabric-dr...-liquid-vinyl/
Also, a video. She has a pattern you can buy, which is linked in the video, but I don't think you need it. It would be convenient, as all measurements given for different sizes. But not difficult to figure out without those.
https://www.youtube.com/watch?v=9m_KcxQe12o