Arrange four 48-inch 2-by-4 boards in a square frame that is 51 1/2-by-51 1/2-inch, with the large, long surfaces of the boards facing down. Drill two holes through each board and into the end of the board that is adjacent to it. Attach the boards of the frame with 5-inch wood screws.
Repeat to create a second square frame with the same materials.
Stand the frames upright on their long, thin sides with the help of an assistant. Keep the frames 20 inches apart. Slide a 20-inch 2-by-4 between the frames in each corner, with their large surfaces facing upward. Drill two holes through each square frame and into each end of these corner boards. Attach the boards with 5-inch wood screws.
Cover the small sides of the bar with 3/4-by-23-by-51 1/2-inch plywood boards. Attach the boards by drilling 2-inch wood screws through the plywood and into the frame every 12 inches.
Lower a 1-by-24-by-54-inch bar top onto the top of the bar frame. Drill holes up through the interior frame and into the bottom of the bar top, being careful not to drill all the way through. Attach the boards with 2-inch wood screws.
Position the portable sink within the rear of the bar. Attach it to the frame using 2-inch wood screws. Attach additional 2-by-4 support boards to the frame to support the sink if necessary based on the design and style of the portable sink.
Run the drain tube of the sink to a little-used area of the ground around the bar, drilling a hole through the bar if necessary to let it pass through.