Pizzabox Flywheel Case

Instead of encasing the flywheel in a vanadium-steel, vacuum-pump equipped chamber, reinforced to withstand 14psi external pressure, encase it in a cardboard pizza-box sealed with duct-tape and use an electrolyzer to keep it full of hydrogen at ambient pressure. Oh, and there should be a relief membrane for when the little friction that is actually produced heats up the hydrogen.

While meant for slow flywheels which can easily shrug off an errant piece of pepperoni without exploding, the rim of even the fastest flywheels can't approach the speed of sound in hydrogen.