Cut two pieces of 27 1/2" x 2" x 2" and two pieces of 25" x 2" x 2". Assemble a frame with the first two pieces running horizontally and the second two pieces abutting the inside of the longer 2" x 2"s. Cut six 27 1/2" x 1/2" x 5"s and nail across the 2" x 2" frame. The last piece will need to be ripped to size (approximately 4"). Cut two pieces of 29" x 1 1/2" x 1 1/2" and nail to the vertical sides of frame. The door assembly should fit in the runners of either bay. Repeat for the other side. Take into account swelling and warping from constant exposure to the elements.