This guide will let you know how to put a lock on your door in The Forest.
In The Forest, cannibals can easily walk through the door, so you will need to put a lock on the door to stop them. You have to go to your book and select Custom walls.
You have to place the wall on the ground and then press R1 to make a window. Now you have a lockable door.
After that, you just need to fill it up with wood logs.