The Trident needs to go on the bottom, and it's two panels high - so it will consume a lot of real estate. I put my single DOS on the top consuming another panel - so that only left one panel in the middle. On that I mounted my 2 ReefWave and Maxspect controllers.
There are 2 cutouts that slip off on each side of the Trident, and they're just big enough to stick a hand through or shine a flashlight around. They're just held in-place by friction so you don't necessarily want to mount anything to them.
So then I have 2 Sicce pump controllers (that I never need to access) and the Apex unit itself (which I almost never need to access), and with my DDR reservoir and the Trident waste container placed beside the AR cabinet, I didn't really have a lot of options but to mount those items inside the cabinet.
I guess I could've mounted the Apex to the side, but it would've either meant snaking all the cables around or drilling out a slot for I will be mounting my apex base unit on the inside.