The thing is, Calico, that your render is excellent with shapes and all, but what you're actually looking for is the complete opposite of what TG2 is doing. Looking at your image, it occured to me that I'm looking at a negative of a real fly-by saturated cumulus. I fly fairly often on my travels, and of course that's a great opportunity to study clouds :-)
I took your render, and created a little mock-up to present my point (conceptually). If I cut out the cloud, invert the image and then up the exposure and gamma, paste it back into the original with the blue sky, I get the following:
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What we need TG2 to do is *darken* the cloud egdes, not to make them lighter than the rest.
Regards,
Frank