When I was little, my grandmother had an old outhouse in the back with a bucket with a bag of lime in it and the chickens used to go in there and lay eggs in the bucket. When I got my chickens, I didn't have time to build them nest boxes before they started laying, so I took a 5 gallon bucket and put bricks in the bottom, then hay on the top and put it in the coop. They loved it so I added a second bucket. I also provided a box with bedding, but I they seemed to prefer the bucket, probably because it was up off the ground. The chickens don't care, but I eventually built them nest boxes anyway.
Oh, don't forget to add a golf ball or fake egg to help them figure it out just in case.
Lisa