This is my Mark II design of a sun shield for my iPhone 5s that I use as a GPS on my motorcycle. As I don't use the phone as a phone and only for this purpose I will be sealing the cover in place using crazy glue. I have a RAM mount 1" ball that I will be attaching to the back as well.

Tickercad link:

https://www.tinkercad.com/things/i17MegjZZgW

If you actually want to use this design rather than using it as inspiration I recommend that you make the phone compartment 1mm thicker and enlarge the radius of the corners a fraction - I had to do some light sanding to make everything fit perfect.