I have been wanting to build a sumo ring for battling hobby robots. Today was the day. I intend on using it to test my own bots as well as a teaching tool and to help out at events at Brickworld. I teach Lego Mindstorms robotics at Harper College through a program called Inzone. The ring is made from a piece of 4ft by 8ft by 1/2in plywood. My dad and I bought it and had it cut in half at Home Depot. Both sides of the plywood were used to form a 4ft by 4ft by 1in thick square. After some geometry to find find centers, we did some jig-sawing to make it a circle. The two rounds are secured to each other by some gorilla glue and 5 screws. The screws come up from the bottom so keep the battle surface free of any imperfections. We did some painting and even made a mask for the white stripe around the outside. If I were to do it again, I would 3D print a tool with my Solidoodle that would basically be a swing arm that attached to a jigsaw. This would ensure that the radii swept by the saw is exactly constant throughout the duration of the arc. In addition, I would also add a mount to attach a razor blade so that the masking and painting process became faster and more exact. That being said, we put a day into building this thing, and it turned out really well!