I had a go at making something similar a while ago, I was combining two cloud fractal shaders one to create a lot of small clouds and another to form these into larger patches.
Here is a small test I made at the time, I have a larger, better quality version cooking at the moment and will post that when it is finished.
There is probably a way to get a more banded distribution of the small clouds using functions. I will give that a try when I get the chance.