[It looks good. If you still have some scrap, I'd put another piece along the back wall to make it even with the ends. Then I'd add a couple of small pieces to the front on the part where it rests on the tank.
That way if the outer box gets too full, it would simply pour back into the tank. This is merely a safety measure.
You will want to install a small baffle inside the outer box, about 3 or 4" tall. The bulkhead will be to one side of it, and the U tube will be on the other side. This will keep the siphon going no matter what.
Remember the U tube needs to be 3/4" off the base of the inside box. And cover or wrap the tube so that it doesn't grow algae in it. A cover is better, so you can lift it to check for airbubbles or a broken siphon. It does happen from time to time, while you get things worked out.]
