Lay out two 3/4-by-24-by-45-inch plywood boards on their long, thin sides. Set them parallel to each other about 42 inches apart and think of them as the side boards of the cabinet. Position a 3/4-by-24-by-42-inch plywood board against the ends of the side boards. Have an assistant hold in place the new board, which is the top board.
Set 1-inch metal L-brackets within the frame, spaced every 8 inches between the side boards and the top board. Mark the screw hole positions and drill them out. Attach the brackets with 3/4-inch wood screws. Repeat to attach another 3/4-by-24-by-42-inch board on the other end of the side boards to serve as the bottom board.
Use the stud finder to mark the studs in the wall where the cabinet will be installed. Position the cabinet where at least one of the side boards is along a stud. Place metal brackets on the interior of the frame against the wall at each point along the side boards and top and bottom boards where they meet up with the studs. Space the brackets every 12 inches along the studs. Mark each of the holes in the brackets and drill into each hole. Attach the brackets with 2-inch wood screws.
Position two 3/4-by-21-by-45 1/2-inch plywood boards against the front of the cabinet frame evenly to be the doors of the cabinet. Slide two side-mount cabinet hinges between the doors and the side boards. Mark the location of the hinge screw holes and drill out the marked holes. Attach the hinges to the door and side board with 3/4-inch wood screws.
Set a three-part magnetic center latch at the top board of the cabinet and the inside of the two doors. Mark the attachment holes and drill them out. Attach the latch to the doors and top board using 3/4-inch wood screws.