That's the kicker with some of these kits. They lop off over an inch of the top of the external box so it will fit traditional rimmed aquariums, but that space is absolutely critical for silence and safety at high flow rates. Basically anything will work with lower flow rates.You don’t want the water level on the outside box too much lower than the water level in the inside box or you will hear splashing goes through and falls. You’re limited by the height of emergency drain.
The one pictured (Exotic Marine) uses spacers to get around trim and a full height external box. I honestly don't understand why more companies don't do this other than they don't "get it."