This paper presents the design of the resistive insert magnet for the 45 T hybrid magnet under construction at the High Field Magnet Laboratory (HFML) in Nijmegen. We describe the design process and tools used to obtain the coil and disk design, and we describe the mechanical design aspects of the housing for the magnet. We address the end turn issue and introduce an innovative new element to provide sufficient clamping of the coil at all times. Finally, we address possible fault scenarios and the magnetic misalignment forces associated with such faults and their impact on the housing.