When I was a kid I made a cardboard bator. Had the top of the cardboard paper box replaced by a piece of plexiglass and suspended a lamp over it.
It was fun as a kid, but the hatch was poor and I ended up with a few messed up chicks that had to be culled. I now know that fluxuating temps can effect late stage development and result in splayed legs and unhealed navals.
Forgive me, as I know next to nothing about building a bator or even bating, but...........Won't the cardboard get kinda mushy from the humidity? And if you are going to all the trouble of building one of cardboard, wouldn't it be worth it to use styrafoam and make something that can be used again? Just asking.