I made a strap extension from an old helmet. Just a straight starp with the D rings. Strap it together like normal, only run the strap under the passenger seat and pull it tight, lock the seat in place. Now the helmet looks as if it's just sitting there, and it's fastened under the seat so it can't be stolen.
I was actually gonna buy a roll of material and a box of D rings and see if i could sell em for $5 a piece on eBay
Hey, it works.