I REALLY struggled with tis as well. Tried all sorts of Ram Mounts (cost a fortune and sitting doing nothing)
I have an Android Google Nexus 5x, so had to get a more Universal case, but they do do specific ones for iPhones normally.
In the end, I got a set-up from Tigra http://www.tigrasporteurope.com/en/
After buying a 2nd mount, this enables me to swap the waterproof case between the Monster and my Mountain bike.
You can also buy an extra battery to put inside my particular case (handy when out on mountain bike)
I got the local Ducati dealer to install the twin USB charger attached to the frame about the engine which works off switched live from ignition. (See photo's)
Hope this helps,.