Actually, interestingly enough, my original reasoning for having such a long canvas was something /like/ that, except with the big similar part down in the bottom- hence all the big flat clouds and stuff. I'm also not really sure how i'd extend the sky- the dark parts of the sky sort of are converging in on the central circle of light-source-ness, and if we extrapolate that upwards we'd end up with a dark sky, so...?
If at all possible, I'd prefer to achieve a similar effect, or at least a unique effect, by lengthening the bottom part of the canvas instead of the top.