Any Material of your choice Assembly: print Caps twice Cut 20mm outer diameter x 18 mm id Metal Tubing to lenght for your bike, taking into account 2mm of flange on each side. I recommend using a roller-type pipe cutter as it slightly flares the edges inward. It gives a very snug fit. Insert the endcaps and press together using a vice. You may have to drill the center Hole a bit larger, depending on your axles.