Anyone else see a flood in the making? First, a overflow box has a box on the outside of the tank, designed to hold the siphon when the water flow stops. This will restart when the water flow restarts. I suspect your system won't, causing your tank to fill until the sump runs dry. Your output needs a anti-siphon hole just below the water line when running to stop the water in the tank from running back into the sump and flooding. Please check out a real overflow box. I made my first from a $8.00 specimen box and PVC
pipe, hardly any flooding.