The ECR source magnet of the FRIB facility has been designed by the Superconducting Magnet Program at the Lawrence Berkeley National Laboratory (LBNL). The magnetic parameters are close to the VENUS magnet (sextupole-in-solenoids) operated at LBNL. The mechanical assembly concept, however, is different and uses a shell-based support structure. The main advantage of this structure is the ability to finely tune the sextupole prestress both azimuthally (by way of bladder and key technology, and axially by means of end plates. This method of structural pre-loading applies the desired compression on the sextupole coils while maintaining acceptable peak stresses. Also, bladder and key technology is a reversible assembly process allowing fast and easy coil replacement, when necessary. This paper also describes the mechanical analysis required to estimate the sextupole and solenoids preloads. The stress distribution in the windings and structural components are presented during assembly, cool-down, and magnet excitation.