Well, that got me more answers than I could have hoped for! :)
GIS shows a whole lot more square clerestories than square cupolas, but
shows both of them in round and square versions. That tells me that,
while the dictionary doesn't nail them down, common usage goes in both
directions, and I can get my message across.
I think the reason that I drew it the way I did is that I grew up in
the Southwest, where the only real architecture was pueblo-inspired
adobe and stucco. I wasn't consciously thinking of it at the time, but
I'm sure that's where that comes from.
I'm toying with building an outbuilding studio thing in my large,
unused back yard, and the L shape above is particularly suited to it. I
was trying to think of some things to do with it to make it less dull
and boxy, which is where the clerestory came in. I have a 45' square
plot separated by a wall of trees from my real back yard to play with,
so I'm having a lot of fun dreaming up stuff. :)
Also, thanks particularly to Michael for giving the alternate spelling,
or else I would have gone the rest of my life--seeing how it's used
commonly in churches and all--pronouncing it "cluh RESS' tuh ree'!"