Two questions.

[MagpieDucks]

Or just close off the nest box that they all want to use and see if that causes them to lay in separate boxes. It might help.

The thing about that is it will most likely just make them lay in different boxes but it won't keep them from laying the same box. Some hens will literally wait in line to lay in the same box as other hens.

 

[JedJackson]

It's a free and easy thing to try. No guarantees, but I'd say it has at least as much of a chance of succeeding as putting golf balls in the nests.

 

[drumstick diva]

Chickens are weird like that, but broken eggs can lead to egg eating. Egg eating is even worse and if they see one hen do it, they all will.