Use a ladder to reach the attic door opening. Locate the insulation dams that frame the inside of the opening. These are pieces of wood that typically sit 1 to 2 inches out from the edge of the door opening.
Measure the length and width of the opening between the insulation dams. Transfer these measurements onto a piece of 3/4-inch thick plywood.
Cut the scuttle hatch out of the plywood along the measured lines, using a jigsaw. Slide the hatch into the opening to ensure it fits properly. Sand the edges to smooth them and to provide a better fit, if necessary.
Cut a piece of rigid foam insulation so it is 1/4-inch smaller than the scuttle door on all sides. Glue the insulation to the top of the hatch door with construction adhesive.
Paint the bottom of the door so it matches the paint on the surrounding ceiling. Allow the paint and insulation adhesive to dry completely before placing the door into the hatch.