Cityfish,
If you want to use the overflow box, it works fine, just takes up space and does not look really that good. Basically, hopefully, it has an adjustment where you can move the inner and outer boxs "up and down independently".
You will have to mess with it a little bit getting the height adjusted. The inner, well, the teeth just above the water line. The outer, uh... It's been awhile since I ran one.... I do know I ran mine a little on the low side to keep from gurgling. The water maintained a constant level way above the outer inlet, if that makes any sense. I had no filter on the outer intake. That just slows down flow for filtering that was done in the sump. If yours has a U-tube, you want the bottom of the U-tube lower than the intake on the outer box, make sense? U-tube bottom lower in the box than the top of the drain tube. Keeps from breaking the siphon, and a heck of a mess.