Use industrial strength 3M adhesives to easily attach your phone to any Helmet, flat or curved surfaces.
I use the curved one on my helmet while riding my scooter. I was nervous, but it seems to hold pretty good. I made sure to clean the surface with a damp rag first, then held it in place for three minutes. I also let it rest for 2 days. So far it hasn't fallen off, even with a couple solid crashes.