I came across this site last night (www.bikehelmetlock.com) & it looks like a pretty neat product (a cable that you run through one of the vent holes on your helmet, which you can then lock to your regular bike lock. It's really just a piece of wire looped into a figure 8 with a big nut in the middle. I have no interest in locking my recently acquired helmet to my bike, but the 'lock' would be perfect for attaching my helmet to my backpack when I walk into work, freeing up a hand.
It looks like the product is no longer being manufactured. I think I could create the same thing at home, but I'm having trouble thinking of a suitable material for the cable. I think that something about the thickness/flexibility of an ipod charging cable would work fine, but I don't want to use one of my ipod cables to make it. To be honest, I'm not even sure where I'd buy a piece of coated cable or something else that would work, aside form using an old USB adapter or something. Mo other thought was using a climbing nut, which would probably work.
Any other ideas? Or does anyone know where to buy one of these?