Overflow box not siphoning

[TheReefGeek]

Sure it can, just make the drain section of your sump small enough that your tank can take the extra water without overflowing. This would be flood-proof in my mind. (yes, anything CAN happen, your baffles could fall over at the same time or the overflow could leak, which is as likely as your tank coming unsiliconed, so it is no more likely to flood than a tank just sitting there with water in it)

You are correct though, drilling is better, I reccomend drilling all tanks when possible. But if you have an external overflow box and are currently using it, might as well make it as safe to use as possible.