I have the Twisty Roads iPhone case fitted to my handlebars, it is superb but mine is an iPhone 6s, they must do one for the plus?
This is the review I wrote on the Twisty Ride website:
"I bought this case specifically for a trip to Germany to buy a BMW R1150GS Adventure and ride it home to Ireland. I bought the Copilot app for my iPhone (great in combination with this case) to navigate home. I contacted Twisty Ride on the Tuesday before I left and told them that I wanted the case for the following weekend. They told me to place the order online and they would ship the case (and charger I ordered) immediately by express delivery. They were true to their word and I received the case, in Ireland, two days later - in time for my trip.
The RAM mount makes it very quick and easy to install and position where I wanted it. It poured rain for the first two days of my ride home, but the case didn't let in a drop of water. In bright sunlight, the screen is a little difficult to see, but this is the nature of the iPhone screen, rather than any fault with the case. The quick release makes it very easy to pop the phone off the mount to take photos, or when stopping at a petrol station etc.
The screen is slightly recessed so when typing, tapping the keys at the very bottom corners is a little difficult. It's fine for typing the name of a place into a navigation app or sending one or two text messages but when sitting down, having a conversation by text message, I found it easier to just pop the top cover open and type directly on the iPhone screen. Having said that, the large buttons on my navigation app were easy to press, even with motorbike gloves on.
The case is a little too large and bulky to use as an everyday phone cover, but it fit easily into the pocket of my motorbike trousers when I was off the bike and walking around.
All in all, this is a great case for mounting your iPhone on your motorbike and keeping it protected."
However, I don't see case for the iPhone 6+ listed on the site, so it's not an option for you.