I've been rendering more sides of your cube, but can't get it fixed yet, sorry
I keep on having little seams.
Pity is that I can't disable GI here because you used quite funny settings for your atmosphere.
In other words: you made it rely extremely on GI in regards to the colors of the atmosphere, but the subtle lighting in clouds isn't there at all since the clouds are not very dense, in a dense atmosphere, and more importantly in 2D.
So by design you could render this one without GI, since it wouldn't contribute much to your clouds anyway.
The only thing you could try is to reproduce the same type of atmosphere and cloud with the usual sane settings. By that I mean not to derive too much from the default settings. Then render that result without GI.
For starters you could use the default atmosphere and tweak the clouds in such a way that they are already quite faint. Then reduce the exposure of your camera and render.
Martin