The Attachment Kit for Helmets has two answers: First, a universal mount for helmets and other equipment that you can simply clamp to the backpack. Second, a toggle-shaped mount that's specifically designed for helmets with large vents, which you can use to attach to the closure strap or daisy chain. That would elegantly solve the helmet question, right?
|Product Name:||ORTLIEB Attachment Kif for Helmets|
|Bag - Parts & accessories :||Mount others, strap / carrying system|