I use a smallish Joe Rocket (cheap) tank bag. If it is too hot I tuck the phone away in my mesh jacket, out of direct sun.
My experience with locations has been the opposite of yours. I find my Garmin more reliable, especially where cell coverage is spotty, and the Garmin Zumo works in the rain pretty well. I blue tooth phone to Garmin and Garmin to my headset. My GPS is also more glove friendly than my Android.
There are spots where the phone navigation is better, heavy tree cover and similar, but generally I prefer the GPS for nav. Most of my riding friends navigate by phone.