If you find a customizable phone case on Thingiverse, you can download the OpenSCAD file and alter it to fit your phone. The software is free, and usually the parameters you want to alter are easy to find. If you have any experience with programming in C, or any friend that has that experience, they could probably figure it out in an hour or so.
OR, you might email the designer and ask them to do it for you. It would help if you sent them the dimensions of your phone, including the location of cameras and buttons.