3D printed doesn’t necessarily mean rigid. There are flexible filaments such as NinjaFlex that are commonly used to 3D print phone cases, rubber tyres for RC cars, etc… Not all printers can handle them well though. For best results, you need direct extrusion, and not something like e.g. Bowden extruders.
That said, respect. Were I to attempt this, I’d probably screw up a couple of cases before I got something semi-presentable. 