This is what Orion Girl told me when I asked about the same topic:
"Floods are a risk, but not inevitable, with overflow boxes. First, there should be 2 boxes, one inside the tank, one outside. The one inside has a variable top so the water pours over even if there's a bit of evaporation, but not so it will remove more than about one inch of water. The outside box has a pipe that limits how much water flows out, so it will always remain above the siphon tubes. So, you should never lose the siphon on this part. On the return tube, you drill a hole just above the water line. If the power goes out, air gets sucked into the tube, preventing the return line from siphoning all the water out of your tank.
Always have flow lines as straight a shot as you can. I ended up needing to replace all the lines, because the lines were angled and air bubbles would get trapped, slowing water flow. It did result in some floods--but not because of the overflow."
I haven't set it up yet, but I intend to. Some of my spare parts bin will be consumed in the process (yay!).
