Late to this thread, but we built our condo upper levels with plywood supported by more cube grid things, and zip ties. No additional wood support needed. Ours are 3wx2dx3h,a bigger setup might need additional support.
We also built “trays” for the whole bottom floor so that I could lift out the entire condo for deep cleaning the litter box level (put the litter on the bottom level). It’s pretty heavy so I don’t do deep cleaning more than once a month. The rest of the time, the 2square tall door was good for cleaning the middle and bottom levels with a quick sweep and there is a one square door on the top over the highest level for easy vacuuming. Also put the whole thing on caster wheels so I could also easily clean behind and under it.
We used vinyl scrap on the two larger levels and carpet scrap on the ramp and top level. Secured carpet also with zip ties and the vinyl we stapled in. Super easy to clean but one of my buns started ripping up the vinyl so she ended up with fleece blankets on her lower level.
I am not home right now but I’ll see if I can drag up pictures from when we were building.