Purpose: Radiotherapy to the pelvis presents challenges as the testes are mobile and exquisitely sensitive to radiation. Lead “clamshells” reduce radiation exposure during treatment, but these devices cause significant imaging artifacts during CT simulation, potentially interfering with treatment planning. We fabricated and assessed the clinical utilization of a custom 3D-printed plastic CT-compatible gonadal shield and immobilization device. Materials and Methods: Our plastic clamshell was designed in Solidworks 3D CAD package (v26, 2018, Solidworks Corp, Waltham, MA) and printed on a printed using a commercial QIDI 3D X-Max printer (2018, Zhejiang QIDI Technology Co., Ltd, China). We evaluated the clinical applications of a 3D-printed plastic clamshell across 3 patients. A Siemens Sensation Open CT scanner (Siemens Medical Solutions, Malvern, PA) was used for radiotherapy CT simulation. All treatment planning and dose calculations were performed using the Varian Eclipse system (Varian Medical Systems, Palo Alto, CA). Results: Our plastic clamshell increased the distance between the radiation target volume and the testes for a 14 yo M receiving proton therapy for fusion-positive rhabdomyosarcoma of the pelvic floor, providing a larger safety margin between the distal end of the proton beam and the gonads. For a 41 yo M with diffuse large B-cell lymphoma, our clamshell successfully eliminated imaging artifact, facilitating more accurate target delineation prior to treatment using a lead clamshell. In a 73 yo M with testicular lymphoma for whom the testis itself was the radiation target, our clamshell served as a scrotal immobilization device, facilitating reproducible setups across radiation treatments. Conclusion: Our custom 3D-printed plastic clamshell is CT-compatible, and its ability to achieve immobilization and shielding has the potential to improve comfort, safety, and oncologic outcomes for pediatric and adult patients. Future work should prospectively assess patient-reported and objective measures of comfort, radiation toxicity, and tumor control.