Here is a cool pet door for interior and exterior doors and hidden litter box installations. The LYNX Cat Door comes with a 4-way locking flap. It has an opening size of 5.9″ x 7.5″ and fits small to medium size cats. You can lock it both ways or only from one side.
The LYNX kitty door is durable and scratch resistant. Doesn’t make a whole lot of noise either.