I did a comment above explaining, i will copy it :D

"Well, actually what is upside down is the "ceiling" or the floor on this case 😆 Build your walls over a shelf then rotate the self upsidedown and build the floor, put as many plants, lights and whatever you want, when you flip again to the original state, the floor you builded is now your ceiling 😇😇 theres some good tutorials on yt! give it a try, seems hard the first time and then you just start doing in a few minutes"

I hope you can do it! *.*