Rather than have it brightly coloured in case you drop it, have a case with a lanyard fitting so you can have a lanyard to avoid dropping it somewhere. I’ve done this for a few years now, and never dropped a phone on the floor when I’m out anywhere.
Trying to fabricate a case that will allow you to use all the controls, charge it, etc, is really not going to be easy, the only way to do it properly is 3D Printing, but there’s a whole bunch of different things to go wrong there, multiple prototypes to get the fit right, for example.
Here’s my phone, in an Otterbox Defender, the lanyard I made up is Dyneema from a marine supplier, it’s used for dinghies and it has a 450 lb breaking strain – same stuff my bowstrings are made from, and paraglider rigging. The loops on the end I stitched with Kevlar thread and put heatshrink tubing over the joints.