Yes, you can have something right on the ground. You can have straw just placed on the ground or inside of a box on the ground. 18" seems a bit high for chicks to jump into when coming to mamma. You can pretty much use anything you want for a nestbox. I have found that a cut 5 gallon bucket resting on the ground with straw or hay in it is sufficient. You could also just leave them on the ground if you think it is safe for them. This way, she won't have to call them to the nest and she can just have a seat anywhere. She will probably pick a spot along the edge or corner of the pen though. Just make sure all the chicks are there before turning in for the night. If you go to a store and get a box for them to sleep in at night, she should routinely return to the "safe" place and sleep there the entire night. I don't suggest leaving the carboard bare underneath the hen's or the chick's feet as it can cause leg/joint problems.