I have never kept rats but the side tank works if you built a heavy plexy bottom 1/3 and a screen top that should work.
logically if a 16" container will fill with ammonia a 6 " will faster. I'd guess that upper and lower holes are needed to vent properly. An open top does not encourage air flow.
I built a plug for a sideways tank, a replaced door with heavy expanded aluminum or steel might be enough with out much of a frame. Or the frame could be on the outside so chewing would be limited by the screen.
Just thinking out loud!
Alex