If you can print with one of the flexible materials you could try what Motorola did with one of the alternate covers for the original generation Moto G - the back cover was hard plastic, but the sides were flexible and wrapped slightly over the edges of the phone front, holding the cover on instead of the clips. This also provided better grip and bump protection.
I haven't found any pics of the inside. The buttons are also easier to do with the flexible material as you don't have to hinge anything.
I haven't found any pics of the inside. The buttons are also easier to do with the flexible material as you don't have to hinge anything.