Cut two pieces of ¾-by-1 1/2-inch popular boards together. Make the length the same as the width of the hatch opening between the walls. Glue the board together to create the 1 1/2-inch square top component of the hatch “ribs” or framing.
Attach a piano hinge to the piece about ¾ inch from each edge. Place a piece of vinyl under each leaf of the hinge and secure with screws. The vinyl prevents moisture penetration.
Rip two pieces for the bottom component out of two ¾-by-3 1/2-inch boards. Glue the boards together. This piece also functions as a surface for installing tail lights and the license plate.
Place a piece of cardboard on the inside edge of the side. Trace the outline of the radius of the kitchen wall, from the top edge of the roof down to the floor. Add 3/8 inch to the radius to allow for clearance between the hatch and the roof.
Cut a 1 1/2-inch-wide pattern with the utility knife. Use the template to create the “ribs” of the hatch. The ribs consist of the vertical pieces that attach to the top and bottom component cut earlier. The ribs provide the structure required to attach the plywood.
Trace the outline of the template on ¾-inch plywood. Use a jigsaw to cut the piece. Glue two pieces together to create one 1 1/2-inch-thick rib. Cut and assemble the number of ribs required.
Assemble the hatch on the trailer. Clamp a rib to each side of the hatch opening with a C-clamp. Attach the top 1 1/2-inch square component between the two end ribs. Use screws, heavy-duty construction adhesive and metal angle brackets to secure the components together. Install the ¾-by-3 1/2-inch bottom component in the same manner.
Install internal hatch ribs at equal intervals between the end ribs. Cut the internal ribs to the appropriate length. Attach the ribs to underside of the top component and to the top side of the bottom. Use adhesive and angle brackets.
Measure the space between the internal ribs. Cut horizontal top and bottom braces out of 3/4-by-1 1/2-inch boards. Stagger the braces between the ribs to prevent lining up any brace end to end. Use adhesive and screws to secure the braces in place.
Secure ¼-inch plywood to the structure. Measure and cut the material. Trim the material when finishing. Clamp the hatch into position on the trailer. Allow the hinges to dry. Apply clamps on as many joints as possible. This ensures the completed hatch has the desired configuration and will not spring back.
Apply fiberglass siding to the interior side of the hatch. Cover all exposed wood. Use the manufactured-recommended adhesive for the siding.
Cut the openings for the recessed tail lights and license plate light in the bottom cross framing. Use a jigsaw to make the cuts.
Follow the manufacturer’s instructions for wiring the electrical components. Hire an electrician to help complete the electrical connections if you lack experience.
Apply 1 1/2-inch rigid foam insulation board in the void between the braces. Glue the insulation to the surface. Cover the interior side of the hatch with 1/8-inch Luan plywood with adhesive. Cover plywood and other exposed wood with fiberglass. Pull the electrical wire through the interior roof material for the roof light.
Attach molding around the inside perimeter of the hatch. Follow manufacturer's instruction for mounting latches or locks to the hatch. Allow the adhesive to dry and cure according to instructions before securing the hatch to the trailer opening.
