You need two very low rpm motors a battery wire something to hold it together(mecano works well) a soldering iron solder hot glue gun
Solder the battery wire to the one motor.
Solder wire from one point of motor to one point of the other motor.
Just follow the diagram and make sure there are two prongs at the front
Glue the motors to each other with the wheels facing in opposite directions
Make a position for the 9v battery to sit on top of the sumo bot
The reason this works is because of the base which is about 30cm wide
The middle was made from aluminium foil and the 5cm ring around it was just paper
I made it a long time ago so it's kind of falling apart.