I've always lock mine, using a homemade, 1/8" SS wire rope / cable, covered with heat shrink, with a loop in each end and a little TSA combo lock. $5-$10, good to go. I've got some hi end helmets & never had a problem with anybody messing with them.
Here in Fl. we have a choice weather to wear a helmet or not. Several of the folks we ride with just leave their helmet sitting in their seat while we all go in for lunch. Never heard of a problem.
I bought the Schuberth HelmetLok....rubber coated and works with all Schuberth helmets....keyless...RevZilla sells them..part # 9049100255..it slips through the helmet strap....I love it ...I attach the helmet to my handlebar and it doesn't scratch at all....it may work with other helmets also. hope you find what you need.
I have a modular helmet that does not have the metal D-clips. I use a cable gun lock that came with one of my pistols. I place the helmet as far forward as I can and run the cable around the handlebars and through the visor. Works well.
I wanted to say thank you for everyone’s help.... I’ve decided on the Masterlock Python cable lock. A couple of reasons. It’s adjustable length.....and mainly..... these old eyes can’t see numbers on a combination lock any more.... (....yes..... I’m too vain for bifocals.......).
I thought about those helmet adapters and the license plate or mirror lock, but it would be too much like putting a jigsaw puzzle together everywhere I went...... I suck at jigsaw puzzles.....