I really like this idea. It always makes me happy to see someone try a top view tank with different fish. Is water going to go to the top of the box, or is that a shelf for plants with water only in the tub?
Walking cats would be interesting. Plecos can be fun from above even though the black limits your options a bit. For other cats a school of pictus, pajamas, aluminium, or pictus cats would be active. You could also go with blue dolphin cats. They are a little strange and expensive but active. Eels like fire, snakeskin or even Asian swamp eels are fun.
As far as cichlids go most don't look good from above, but I have seen ponds of EB acaras that were pretty cool. I also had a friend that did tropheus from above and that was kind of cool. I know you didn't want a solo fish, but I think a Fahaka would be interesting.
I also think a large school of butterfly fish would be fun. An albino clown knife would be cool. A school of pink tailed chalceus would be interesting too. A lot of people haven't seen them but there was a blushing and long fin version at one time. I don't know if they are still around though.
I have seen rope fish in a top view tank that I thought were fun. You would need a lot of plants for them to rest on up high. You would only see them occasionally though. Polys can work.
I also really like turquoise rainbows from above I once saw them in a pond with hatchet fish and thought that was really cool. Pike live bearers would be something different. If you have never seen a fancy guppy pond that is pretty amazing as well.
If you don't mind going brackish anablebs or archers would be really neat. You could keep some monos under them. You could even do a Colombian shark under them for your catfish. Mudskippers would be cool in there, with the shelf to run around on. You have room to plant mangroves in it.
It should go without saying but I am not recommending all of these together.