Nice looking tank and good job on that box!
My main tank is a full external beananimal (no inside the tank box) and also runs three 1.5'' drains that are very strong and braced with hangers. Always brace your piping, next time you start and stop your system, pay close attention to your drains and see if they wiggle as they attempt to purge the air. It might be important to note that this project is a addition to the existing system that will receive maybe half the flow of the main system so it should only require a 1'' drain and a 1'' emergency. (30g) cube.
The return pump of the main system is "T" off with two returns feeding the main and a third feeding the cube with both drains (main siphon/cubes main) draining into the same sump chamber. The gate valve on the cube's 1'' main drain seals the deal for equilibrium. This is how i did the hydroponic flood tray i was using to propagate my nems.
Toothless would be nice but i could never go that route, i have a sixline that enjoys cruising the front of the weir, two peppermint shrimps that will occasionally go up there at night to snack on bits of algae after they've cleaned out what was left on the algae clip. HAHA! crazy shrimps, they eat all the aiptasia in my tank except for three then develop a taste for nori instead, better then the last pair that decided corals tastes better i guess.
Then there's the joy of chasing a lone unfortunate fish around a crowded overflow box My stand pipes are actually not glued for easy access for cleaning.
A turbo/s snail, snails in general that wanders in there and works their way into the drains getting stuck, next thing you know you have half your drain system torn apart stabbing at it with a metal coat hanger. True story, actually happen to a member here (sorry Zach) i think its was him anyway.