Set the cardboard tube form on the plywood so it is standing up and trace two circles on the plywood the size of the end of the tube. Trace the inside of the tube and the outside of the pipe. Locate the center of the cardboard circles and set one of the 1-inch iron pipes over the center and trace a center circle.
Cut out the cardboard circles and the center circles using a jig saw. Cut the cardboard form to 24 inches long.
Weave the 1-inch iron pipe that is 26 inches long through one of the plywood circles. Set the plywood and pipe inside the cardboard so the pipe protrudes out of the plywood circle 1 inch. Set it on the ground with the plywood down, but keep the pipe sticking out 1 inch. The plywood circle should fit perfectly inside the end of the tube.
Mix the concrete. Fill the cardboard form with concrete. Fit the other plywood circle on the top of the form so the pipe is sticking out 1 inch through the center of the plywood on top. Let the concrete cure for at least 2 days.
Remove the cardboard form and plywood circles. You will have a concrete cylinder with a pipe sticking 1 inch out each side.
Weave one of the 26-inch threaded iron pipes through the 1-inch pipe through the center of the concrete cylinder. Attach the elbows on each end of the pipe. Use a pipe wrench to tighten the elbows. Make both elbows point the same direction.
Screw the 4-foot-6-inch pipes into the elbows. Screw the elbows on the ends of the pipes and connect them together using the last 26-inch iron pipe.