Placing decoys like ping pong or golf balls in the nest boxes is a good idea. If they still lay stray eggs, you may need to keep them cooped up in the mornings until they are done laying, before letting them out and give them no choice but to use the nest boxes provided. Make sure the nest boxes are comfortable, clean, semi-dark and attractive for them.
Free range birds sometimes need to be 'trained'(or re-trained) to lay in the coop nests, especially new layers. Leaving them locked in the coop for 3-4 days can help 'home' them to lay in the coop nests. Fake eggs/golf balls in the nests can help 'show' them were to lay. They can be confined to coop 24/7 for a few days to a week, or confine them at least until mid to late afternoon. You help them create a new habit and they will usually stick with it. ..at least for a good while, then repeat as necessary.