The rapid deployment of fifth-generation (5G) communication systems has accelerated the growth of IoT, while simultaneously intensifying electromagnetic interference (EMI), which adversely affects sensor stability and measurement accuracy. To address this challenge, we propose an automated cylindrical electromagnetic shielding enclosure operating in the low-frequency 5G band, designed to provide an interference-free environment for sensor testing and quality control. The enclosure is fabricated from aluminum alloy and designed to meet IP68 sealing requirements, with a tapered contact interface to minimize signal leakage. To achieve high shielding effectiveness, O-ring-shaped microwave-absorbing composites were developed by mixing ferrite powders of Fe3O4 with epoxy resin at weight fractions of 15, 20, and 25%. In addition, the hybrid arrangements of copper sheets with one-layer, two-layer, and three-layer aluminum foils were also used to investigate the EMI suppression. EMI shielding performance under different compressive pressures was experimentally evaluated using S-parameter (S21) measurements obtained from a vector network analyzer with unidirectional antennas. Although further improvements in broadband EMI shielding performance and high-frequency applications remain desirable, the proposed design addresses practical challenges associated with signal leakage and shielding stability by integrating automated pneumatic compression, ferrite-filled microwave-absorbing O-rings, and multilayer metallic shielding structures. The results demonstrate that the proposed shielding enclosure achieves high radio-frequency signal attenuation performance and effectively suppresses EMI, providing a stable and reliable testing platform for sensor-oriented IoT devices in 5G environments. Unlike recently reported cylindrical shielding enclosures that mainly rely on metallic shielding structures, the proposed design integrates automated pneumatic compression, ferrite-filled microwave-absorbing O-rings, and multilayer metallic shielding to simultaneously enhance shielding effectiveness, attenuation stability, and testing repeatability.