Are you looking for something that will comfort you, or for specific suggestions of actions to do?
In my case, when I'm having a meltdown, generally I'm feeling unloved, so what would help for me is a list, written by my loved ones, of things they like about me.
If you're looking for specific actions, though, you could try things like 'count to 10 while taking deep breaths' or a calming place you can go.
Either way, it would really have to be individually tailored to you. What causes your meltdowns? What emotion, if any, are you feeling during a meltdown? (I say 'if any' because sensory overload is not an emotion - just like surprise and confusion aren't emotions. They're cognitive states.) Have you noticed anything that helps, even sometimes?