I have two sides of my flexariums covered and the MistKing nozzle in the other front corner angled inwards and it is working fine so far. Occasionally I'll have a little damp spot in front of the one cage, but usually that's because of the angle of the nozzle has shifted cause it's fallen a little bit (slipped down in mesh cause I don't have it attached securely)
That being said, I have two suggestions you could try if you're worried about completely covering sides, especially with summer coming up ventilation could be more of a concern.
Firstly, although not a terribly attractive idea, if you angled the side coverings out like a V - so the water would still be caught and slide down if it went out of sides, but it's still partially open at the top for circulation rather than straight close against the sides? If your decor is modern-art-ish, the angle thing might blend? Lol
Otherwise, what about Marty's new rain-dome cage top nozzle contraption? That would likely keep the water contained better, no?