I assume you mean a chick door? A pop door is the chicken entrance/exit in a coop.How do I make a pop door?
For chick/panic doors, it really depends on your set up. I've seen them made from cardboard, wood... but I use chicken wire, and a wire closet shelf.
Chicken wire version: make a fence, prop it up on bricks (not propped up here but you can see the bricks). That makes gaps high enough to fit chicks up to about 8 weeks old.
Wire closet shelf: this only works for me because the roof of my brooder has ridges that the shelf can "hook" onto for stability. The openings created are about 2-3" wide, maybe?