Andrew Janjigian on Instagram on backing up your starter:
I am super paranoid about losing my sourdough starter, which is why when I refresh mine, I place the previous one in the fridge in case of catastrophe (I also keep a backup backup in my fridge at work, just in case; I told you I am paranoid), then I throw out the previous previous one. I suggest you do the same here, so you can at least go back one generation if need be. (Which is why you'll need two containers to go back and forth from; be sure to clean them thoroughly between uses.)
I follow a similar pattern, except I go a little bit further and keep three backups in the fridge. I also put a little piece of tape on top of the mason jars with the date it was backed up.
Have I ever needed to use a backup? Yes— twice. Sometimes a starter can just go wonky on you.
P.S., you can find those fancy freezer lids on Amazon, but they are a bit pricy right now. You might have better luck in a local grocery store.