As seen in the General Discussion section about securing a helmet to the bike...
I'm positive this idea is not new, but it's new to me.
Yesterday when "Uncle Dave" and I were touring the Wisconsin Alps, we made a few stops that called for shedding the Aerostich suits (sunny and low 80's). Not wanting to take the time to fold up the suits and shove them into luggage (Dave) or not having the luggage mounted for the trip (silly me), Dave pulls out a 6' long vinyl covered steel cable and padlock. He captured the cable onto his bike, threaded it through our suits (down through an arm) and helmets, and captured it back onto the bike again. This is not a long term or high security measure, but it is fast and keeps honest people honest. His cable had a red vinyl cover for added "see me" value so passersby can tell a cable is there and unless they have a cutter with them, they'll know not to touch.
Here are some photos of the cable I made last night. All supplies are available at your local hardware store. Bulk vinyl covered cable, cut to length at the hardware store, will cost you about $.50/foot. My local guy was already closed at 6:00pm so I bought a 20' bulk pack (clear only, no colors) from my local big-box home center (Menard's). Little aluminum sleeves to make loops are about $.99 for a 2-pack. And I bought some fresh brass/steel Master pad locks, a 4-pack all keyed alike. (Now my Pelican cases and the security cable(s) will all be on the same key.) My total cost for this project was $20.
I cut a 7' long piece of cable (cuts easily with lineman electrical pliers if you don't have dedicated cable cutters), used about 5-6" from each end to make the loops with the aluminum sleeves, the result being a 6' long security cable.
Coil it upon itself, slip the lock shackle through the loops and capture the coils to keep a tidy unit.
Close-up of the sleeves to make the loops. You are supposed to use matching size sleeves to the cable, and it says if using vinyl covered cable to strip the cable to get it through the sleeve. I wanted my loops to be vinyl covered, so I cracked open the packages of bigger sleeves and found one that would accept the vinyl covered cable. My cable size (w/o jacket) was 3/32", and the sleeves I used were 5/32" which slipped perfectly over the vinyl jacket. Slip the cable through the sleeve to make the loop, then put a wide slotted screwdriver set into the slot recess of the sleeve and then give it a couple moderate whacks of a mallet or light hammer to crimp the sleeve down on the cable. Do this from each side of the sleeve. Don't bash too hard, this is just aluminum. Electrical tape around the sleeves will help prevent accidental paint scratches to your bike.
The passenger grab rail makes for one of the best capture spots on the bike. Use any similar capture point on the bike.
Here is the cable set through my helmet. No chin bar? Grab the strap D-ring. Yeah, a 6' long cable to secure the helmet only is overkill, and the helmet could fall off the bike seat to the ground (if that's where you set it, but it would be the perfect length if you set the helmet on the ground), but with my 20' roll of cable I bought, I can easily make a couple shorty cables (22-24" piece, put loops on each end, results in a 12-14" long cable, perfect for a helmet resting on the seat).