Have you cut the opening for the door yet? If not, buy a plastic cutting board (Walmart?) approx 12"x18" and enough furring stips to make 4 pieces about 36" long. Cut an opening in the wall about 12" high and 1" less than the width of your cutting board, wide. Plan on having the bottom of the door be about 3" lower than the wall opening so that critters can't get hold of the door and lift it. Attach a furring strip to each side of the opening, about 5/8" from the edge. This will make the furring strip opening 1/4" larger than the door. Then attach the remaining 2 furring strips to the first strips, with the inside edges the same as the inside of your cut opening. This will form a "U" channel for the door to slide in. Now attach a piece of wood 3" below the opening for a door stop, connect rope and pulleys and you're done. Plywood could be used for the door instead of plastic, but the plastic won't warp or swell the way plywood might. This would have been so much easier with pictures. Hope it's all clear.
Paul