A 3D printed cast or splint for application to a patient's body to immobilize a body part includes first and second shell portions configured to conform to at least a portion of the body part. The first shell portion has a distal end, a proximal end, opposing interfacing edges, a plurality of venting holes and a first engagement mechanism positioned proximate one of the opposing interfacing edges. A reinforcement portion extends between the proximal and distal ends. The second shell portion has a distal end, a proximal end, opposing interfacing edges, a plurality of venting holes and a second engagement mechanism positioned proximate one of the opposing interfacing edges. The first engagement mechanism interacts with the second engagement mechanism in a mounted configuration to facilitate mounting of the first shell portion to the second shell portion.