Looks really clever.
But why do all solutions leave the helmet hanging? For obvious reason it will hand upside down and be at the mercy of a shower, unless the local wino barfs in it.
A solution where the helmet rests on the seat (or the tank), with a D ring on one side and a lock for the helmet's D ring on the other and it would stay there, and would be decently protected from the elements, should they unleash their fury