Cut four of the 4-foot by 4-foot cardboard slip sheets into eight pieces of 2 feet by 4 feet. Local warehouses or grocery stores may give these slip sheets away for free.
Cut the other slip sheet into twelve pieces of 8 inches by 12 inches.
Layer three of the 8-inch by 12-inch pieces of cardboard on top of each other and secure them together with hot glue. Repeat this until you have four chunks of layered cardboard. These are the supports.
Drill three holes, evenly spaced, across the shorter end of each 8-inch by 12-inch chunk of layered cardboard, about 2 inches down from the edge. Make sure the holes are measured precisely for each piece.
Layer four of the 2-foot by 4-foot pieces of cardboard on top of each other and secure them together with hot glue. Repeat this with the four remaining 2-foot by 4-foot pieces.
Measure the thickness of the 8-inch by 12-inch chunks of layered cardboard supports.
Use the measurements to cut four rectangular holes, evenly spaced, on both of the 2-foot by 4-foot chunks of layered cardboard. The holes should be as wide as the thickness of the supports and 1-foot long, with 3-inch margins.
Push three of the dowel rods through the drilled holes in two of the supports. Repeat this with the other three dowel rods and two supports.
Push the bottoms of the supports into the pre-cut rectangular holes in one of the 2-foot by 4-foot pieces.
Place the other 2-foot by 4-foot piece on top of the supports, pushing the tops of the supports through the rectangular holes until the top shelf is resting against the dowel rod supports.