I just took pieces of 2 X 4, nestled it down in the wood shavings for support the narrow side up and let the birds do what they wanted with it. As they got older I used long pieces of 2 X 2 braced at the edges of the brooder. They still slept on the floor, but liked to roost occassionally during the day.
Same thing when they were let loose in the coop. They spent the first week or so on the big roost during the day but slept on the floor. Once one started actually roosting at night, the others followed.