WOTOW Folding Bike Lock, Heavy Duty Steel Foldable Bike Chain Lock with Mounting Bracket, 35” Unfolding Anti Theft Bicycle Lock with 3 Key for Mountain Road City Bikes Scooter Motorcycle

Do not use a cable lock for anything, ever. Guaranteed stolen eventually, especially something as unique as a Onewheel. If you need compact and relatively light, a folding lock like this would work, this is the one I use the most often: https://a.co/d/08NBZ7wu But for better security (and probably overkill for your situation), you may choose a chain lock (whether this fits through your rails depends on your model of Onewheel - fine on XR and GT, not fine on Pint probably): https://a.co/d/02WSq2Td If there are lockers available to your kid, they may consider storing the lock in there for weight savings while floating.