So here's what I got...
[attach=1]
As you can see, the distribution shader arranged that way, does some rounding of the cloud edges. It's good as well, but you probably want to expand the cloud mask then...
However, the distribution shader does nothing to alter the look of the cloud base. It is still a featureless pancake...
In a way, I can understand that - any massive cloud like this one is bound to block a lot of light coming through.
Now, I'm not sure if I connected the nodes the way they are supposed to be to achieve the effect. So if you think it should be arranged differently, let me know.