I actually had thought of that too. For my run/tractor especially, I could use those. I thought of spray painting them too, to make them last a little longer...
Just wanted to say something . . .
with milk cartons be aware of toes and shanks falling through the holes
and
with cardboad boxes you should change so they dont get soggy.
I have bought two used above fridge type kitchen cabinets 12"deep, 15" high, 30" wide. I intend to remove the doors, create a bit of a small lip to keep bedding in and use it as a small community nestbox. I will make a hole in the back to that I can get to the eggs from the outside, through a hatch in the siding of the coop. The cabinets were $10 each at restore.