Reportedly, the locking gas cap from ScootUSA works with our stock teardrop tanks.
www.ScootUSA.com Scooter* Gas Tanks and Caps
Won't do too much if your fuel line isn't properly secured, but many vandals probably wouldn't think to pull out the fuel line in the heat of the moment.
The good thing is gas costs next to nothing for us! I can go a whole week without a refill!