I tried a cardboard carton once and it was a soggy mess. Won't do that again. I've also tried styro cartons a couple of times and I had a couple of chicks that struggled with popping out of the shell because of having to hatch "up". I've gone back to laying the egg on it's side. The chicks seem to have an easier time kicking out, even if they do play pool with the unhatched eggs.It makes the humidity toooo high. I do not use the cardboard cartons, because it is a wet mess. I also do not want my eggs wet.
Just my opinion.....
Yes, I think the strong opinions around this topic are why folks get hesitant to offer their advice. No one wants to step on anyone's toes. Some shells are just so thick and I watched a special that Cheryl Cohen had linked once that showed a mother hen helping the babies out by pecking at the shell to loosen it. I figured if mother hens sometimes helped their babies out then I could too. lol
When a chick slows down during the hatch, I find if I tap on the side of the incubator with my finger nail, it seems to throw the chick into overdrive of zipping. I assume they instinctively hear it as Mom.
Deb