Overview
Designing the case of a motor is a critical step in the road to a custom propulsion system. A critical step in this process is choosing the correct material.
Considerations
Mass - If you're planning on flying the motor, weight is bad.
Ductility - ductile materials fail safely (they don't produce shrapnel)
(6061 H case after an overpressure)
Documents
Here are some documents on how to design a motor case.