The cat entrance was constructed totally from scrap materials.  The unit is constructed in two parts: an interior section that fits precisely into the window frame similar to an AC window unit and an exterior box that attaches to the interior section with four large screws.  The assembly is not fastened to the house and can be easily removed by removing the box-lid and removing the four large wood screws holding the two sections together.  The only modification to the house was a 4” x  7” section cut out of the window screen.  The outer section contains a free swinging door.  The small wooden blocks on the box floor are to discourage the cats from sleeping in the box and blocking the swinging door.  The exterior section is well painted with oil based enamel - both inside and out.