I used 2x2 fencing, bungee and a flannel pillow case for mine. I left both ends open for easy flow of chicks from both ends so none could get trapped and smished and adjusted the height of the brooder mama to fit the size of chick...short for new chicks, a little taller for week old and above.
New chick level, with the heating pad touching their backs and they have to squat and crawl under the frame a little ....same height front to back like a mama hen.
Older chick height...again, same height front to back.
By the third week they are mostly sleeping on the top during the day and some under, some on top at night. And that's when the weather was still 30s-40s at night, 40-50s in the day.