I had a hard time finding a container small enough to support them all - mason jars were too big but an egg cup was too small. The measuring lid off the laundry detergent proved to be the perfect size. We put it near the window that gets the most sun, which is not much given we're in the Rocky Mountains and it's winter. Right now it gets 3 hours of sun in the morning... if it's lucky (Last week this window froze over and the onions are still growing!).
It seems to be working, though, because we've had them for a month now. And now we just chop off only as much as we need for a recipe (which isn't much for just the two of us) and let the rest keep growing.