I have 2 BO and 2 RIRS that are 16wks Old. I had them in the house up until 5 days ago. I'm currently coop training them for another 2 days,as I read to coop train for 7 days, then I'll open the coop door to allow them in their run area. The nest boxes are closed off but I don't know at what point do I allow them access to the 2 nest boxes and what do I put in the nest boxes that will be easy to keep clean?