Hi again!
A very short (and possibly poor) instructable on how to make a simple, yet exclusive cat door. I will do my best to answer any questions though....

Step 1: Materials and Assembly

You will need:
-cheap wood for the inner frame
-nice wood for the outer frame
-a thick piece of leather for the flap/door

The picture pretty much shows the construction. Start with the inner frame by glueing a piece of leather to the top piece of the frame ( ideally glued in a cut out slot made with a router- forgot to take a photo of that-sorry). Use screws to assemble the inner frame.

Oh-and note that the thickness of the inner frame should be the same or slightly less than the door/wall where you mount it.

Step 2: Mounting in Door/wall

Cut out an opening in the door/wall where you want the cat door. It should be a few millimeters larger than the inner frame. Just put the cat door there- no mounting required as the outer panels will keep the frame in place.

Mount the panels to the inner frame using small screws. The panels pressing at the door should hold the cat door in place.