If you need a competition legal flywheel equivalent you could mount short steel axles to a wheel or large gears with rubber bands and/or zipties. Just make sure they are securely attached and don’t turn into projectiles at high speeds.
But if it is not for the competition then using tire or fishing weights could become a fun engineering challenge of creating a robust flywheel balancing procedure.
Cool project!
Has he tried already to build a seismograph turning sound or vibrations into a graph?
