Just need to establish if you both have arms or not before I make a suggestion, don't want to offend and all that ...
If it's "safe" to do so then I use a shotgun lock that I keep under the seat. You put the chin guard on the rear passenger peg so the helmet is hanging facing up, the shotgun lock then goes round the chin guard and through the small holes on the foot hanger. You can then push the padlock into the helmet and close the visor over the cable.They're only £2.50 on ebay and are just long enough to do the job.
a strap on helmet??? that's new,