Funny you should say that, lost a helmet at the local walmart, cops brought it back the next day after catching a 14 year old on a scooter.
It is going to be tough to not have a second key. I see locks on E Bay and others but you will have a second key. A trunk would be an option but lots of money and still a second key. My helmet lock for my 1100 was also unavailable. I would just put the helmet over the mirror and tightened the chin strap. That would at least mean a person could not just walk by and pick it up. My Stratoliner has a cable that you hook under the seat and through the helmet. I just do not bother to use it. It is easer to hook it to the mirror. Sometimes I just sit it on the seat. In five years and 55k miles no one has bothered a helmet.
Unless you have an expensive option in your helmet most people would not want it. I suppose after saying that someone will probably loose a helmet to a kid with a mini bike.