It's taken a while to think of something that would benefit someone with limited hand mobility that:
a) isn't super complicated

b) isn't cumbersome

c) is created for the purposes of creating something rather than addressing a specific need

Supplies

3D Printer

PLA for the printer

Step 1: Taped Up Hands

I spent a morning with taped up hands, and went about my day, and by
far the trickiest thing was getting to grips with my iPhone 6 Plus. I love my iPhone and not being able to use it well made my face sad :(

Step 2: The Solution

Behold, the solution to (at least my) problem, the ergo iPhone case!
With the easy grip side handle you're able to hold the iPhone easily whilst using your other hand the operate it. Not only is there a side handle, there are also 3 finger holes to allow customised gripping The iPhone case was designed from scratch, starting with the Apple official guidelines for accessory design. This case adhere's to all of them so buttons are easily accessed, the camera is usable and no connector sockets are obstructed. Even 3D printed this is a perfect snap fit. So did i address my initial questions? yes, yes i did

a) It's super simple, elegant design. no moving parts, no assembly.

b) it hardly adds any weight and is barely bigger than the iPhone itself but is super durable

c) It was designed to meet the requirements of one problem, not a jack of all trades device.

iPhone 6 Plus is big! but now it's accessible to anyone regardless of their hand moblity (i'm keeping mine on my phone BTW!)

Step 3: 3D Printing

I printed me case in a splendid bright orange PLA
Settings were

Layer Res - 200micron

Print temp - Layer 1 - 220C

all others 200C

HPB - 52C

Speed 70mm/s

Infil - 100%

NOTES: This is designed for the iPhone 6 Plus, other variants can be designed upon request I am right handed, so i have designed the case to be held in my left hand - A left handed version can also be designed upon request