Place two 1-by-6-by-20-1/2-inch boards flat on a work surface. Find the center of the 6-inch width of the boards with a tape measure in two places and mark them with a pencil. Using the two points to align a straightedge, draw a center line down the entire length of the board with a pencil.
Measure in 3 1/2 inches from each end of the two boards and mark the spots on the center line. Measure 4 1/2 inches farther in from the 3 1/2-inch marks and mark those spots.
Drill a hole at each mark with a drill press, using a 4-inch hole saw bit.
Place the 1-by-6-by-20-1/2-inch boards flat on a table saw, one at a time. Measure and mark 2 1/2 inches in from the sides on each end of each board; the mark on the ends should coincide with the center line on the face of the boards. Saw the boards lengthwise at each measurement on the table saw. You will end up with four 2-1/2-by-20-1/2-inch boards that have four semicircular cutouts each. The cutouts are for the wine bottles to rest on. These four boards are the sides of the rack. Sand the cut edges of the sides with a fine-grit sanding pad until they are smooth.
Position two 1-by-10-by-10-3/4-inch boards on a work surface parallel to each other. Space them 20 1/2-inches apart on the inside edges and standing on their narrow sides. These are the ends of the rack.
Set two side boards in front of and two behind the rack ends. Position the side boards with the semi-circle cutouts facing up. Locate the two top side boards down 2 3/4 inches from the top edge of the ends and flush with their sides. Locate the bottom two side boards up 2 1/2 inches up from the bottom edge of the ends and flush with the sides. Drive two evenly spaced decorative-head wood screws through the end boards into the ends of the side boards at each joint with a screw gun.
Rub two to three coats of tung oil onto the rack assembly with a cloth rag. Allow at least four hours between coats. Allow the final coat to cure for 48 hours before use.