I designed a new, super slim cover for my Motorola Moto X4. Its shell is only 0.8mm thin! It got a pocket for a 40mm diameter magnetic mounting plate.
It fits absolutely tight and once it is mounted on the smartphone, you can not disassambly it, without destroying the shell. No Problem, a new one takes only 40 minutes to be printed.
Print it flat without supports.
Play with the perimeters extrusion width to get a nice result.