The “phone lanyard” link from stan’s first response is exactly what I got for our Africa safari tour, so that I wouldn’t fumble the phone while taking pictures of lions. elephants, etc. It worked - I didn't drop it out of (or inside) the Land Cruiser!
I also have a silicone case that covers the back and sides of the iPhone, with an open slot at the bottom that the tab with the ring for the lanyard clip fits thru. Without the lanyard now, the ring flips up and gets in the way of the charging cord slot, so I’m always having to dig the ring out of the slot space to pivot it out of the way so I can insert a charger cord, but that’s the only drawback for having a good way to keep the phone attached to me. The trapezoidal adhesive pad that sticks to the phone doesn’t absolutely need any cover or case of any kind to keep it stuck to the phone, but my case has saved the phone from breaking when it’s gotten dropped or knocked off a table or shelf, so it’s always on the phone, along with the lanyard ring stuck onto it.