The Avocado Phone Case aims to be an easily customizable phone case. 

It was designed using OpenSCAD with the Gigaset GS290 / Volla Phone in mind, but you should be able to measure your device and get a working model. It even works for tablets.

Customize this model using Cora or have a look at the source code repository, where you'll also find the latest project files and a couple of templates for use in the OpenSCAD desktop app.

This model is meant to be printed with TPU or any other flexible plastic, you might be able to get away with harder plastic, but might have trouble getting the phone in/out.