Wooden plank (we used poplar) that is 1/2-inch thick, 6 inches wide, and 39 inches long
Carpet remnant (ideally from the room in which you will use the cat scratcher), cut to the length and width of the wood plank
Weights (canned food works well)
Drill with 2-inch hole saw attachment
Paint (to match the door, if possible)
Optional: heavy-duty Velcro, catnip
Door Cat Scratcher How-To
Glue remnant to board with wood glue. Place assembled board and remnant face down, and place a weight on it. Let dry.
Measure from the floor to the center of the doorknob. Subtract 1 inch. Measure this length up from the bottom of the board and mark with a pencil. Using a 2-inch hole saw drill attachment, drill through board and carpet, with the top of the hole meeting the line you have marked. Note: Make sure the hole is big enough for your doorknob; 2 inches works for most.
Add 3/4-inch-wide lattice strip trim to all four edges, using wood glue and finish nails. The lattice can be painted to match the door, if desired.
Optional: Attach the bottom of the cat scratcher to the bottom of the door with heavy-duty Velcro, to keep the scratcher from swinging. Cats can be enticed to use the board with a little catnip until they get into the habit.