Set two 3/4-by-26-by-36-inch plywood boards on their long, thin edges with the help of an assistant, spaced about 20 inches apart. These are the sideboards. Put one 3/4-by-20-by-26-inch plywood board perpendicularly against each end of the sideboards.
Set 2-inch metal L-brackets inside the frame between each pair of frame boards, spaced every 6 inches. Drill into each bracket hole and attach the brackets with 3/4-inch wood screws.
Set a 3/4-by-20-by-37 1/2-inch plywood board on the ground to be the rear board. Set the frame on top of the board and position 2-inch metal L-brackets every 12 inches against the frame and the rear board. Drill out the bracket holes and attach the brackets to the boards with 3/4-inch wood screws.
Position a 3/4-by-20-by-36 3/4-inch plywood board on top of the frame, evenly spaced. Slide two side-mount cabinet hinges between this board, which is the door board of the cabinet, and one of the sideboards of the cabinet, which are the 36-by-26-inch boards. Drill out the holes for the hinges in both the door and the sideboard. Attach the hinges with 3/4-inch wood screws.
Position a magnetic latch against the other sideboard of the cabinet and the door board. Drill out the holes for the latches and attach them with 3/4-inch wood screws.
Set the cabinet upright and position a 1-by-20-by-28-inch countertop on the cabinet, with 2 inches sticking off the front of the cabinet. Drill eight holes up through the cabinet and into the countertop. Attach the countertop with 1-inch wood screws.