https://www.etsy.com/ca/listing/185427326/6-wooden-nest-eggs-to-encourage-chickens
These have always worked well for me.
I use these wooden eggs too, got them at Hobby Lobby. They are VERY realistic and am not entirely sure that the chickens don't know they aren't real. I put them in the nest boxes before they are due to start laying and it has worked so far on the hens I got last year. I presume, by putting them in the nest boxes prior to their first laying that they pecked at them and gave up when they found that it didn't do anything. What I am getting at is, it basically trained them. And I just keep them in the nests, because I see no harm in leaving them there. They are so realistic in fact that I have gone to collect eggs and had to go back to the coop to return the wooden egg.