Lachesis, I forgot to mention that it is best to hang the door piece from the top-center of the hole, and not on the outsides.

Also what tools you will need: a screwdriver, pencil, a square, ruler, a key-hole or equivalent electric saw, and (electric) drill. As far as making the door piece of something transparent you may also need a hand file to file off the edges of the plastic, or an electric sander will also suffice, so they won't cut themselves should any of them decide to back out once entering. Cats do this sometimes. This is the reason why I suggest that the door material should be transparent to help eliminate this problem.