Optimized for Semiflex/Cheetah. Ninjaflex will probaly work, too.
Some softer types of pla may work, but the hinge can break eventually.

Customizer doesent seem to like the imported stl, so you may need to download the .scad file and use Openscad locally to customize (its free!).

Customizable types: Wallet Case, Flip Case, Bumper
Customizable text: You can add your own name or signature in customizer, by default its "OnePlus"

Default settings:
Wallet Case with 3 credit card pockets, one extra pocket and an integrated stand.

We all suffer from bad bed adhision from time to time, why not use it to our advantage?
When adding a small air gap between layers they still adhere during printing but can be seperated to form pockets :-)
I used Simplify3D, not tested with other slicers.

If you have problems separating the layers add a small piece of tape at the air gaps during printing. Usually its enough to have a 3mm wide opening to insert a screw driver and succesively open the pocket. I used a piece of kapton tape for the stand only, you can see it on the picture (yellow tape). If you use too much tape the layers may separate during printing which can cause deformation or a failed print.

You should use a flexible material for flip and wallet case styles, as the hinge can break after some time with harder materials.
Optimized for Semiflex/Cheetah, but sucessfully used Taulman PCTPE (which is a combination of Nylon and TPE=Ninjaflex), too.

For Taulman PCTPE use following settings:
layerHeight: 0.1
standZLayer: 5
caseSideThickness: 1.9
caseTopBottomThickness: 1.9
phoneDimensionAdditionalX: 0.1
phoneDimensionAdditionalY: 0.1
phoneDimensionAdditionalZ: 0.1