You could pop some normal eggs under her until she's happy and then swap out for the fertile ones. Also, don't worry if she ejects any from the nest. It will be no good and she will know. This might happen latter in the process, I guess from smell. Clever chickens.
Also, if there are any other hens with access to the nest, you need to mark the fertile eggs as she will somehow magic other eggs into her clutch. We have no idea how they do this. Last year we were tempted to set up CCTV but I'm sure it would just go fuzzy at the crucial moment.