[tbittner]

The top of the box isn't important, it's the top of the baffle that will keep water in that compartment that's important. The water level in that compartment needs to stay high enough to keep the end of the tube under water.

You can try to lift the box up a few inches, which in turn will raise the top of the baffle.

Or you can make a new U tube that is long enough to reach close to the bottom of the compartment with the baffle. That will work too. Just know that when the pump is turned off, the siphon will lower the water in the tank to the same height as the top of the baffle in the outside box