This paper demonstrates a large-area, lightweight, conformal plasma device that interacts with propagating X-band microwave energy. The active elements are rugged plasma-shells - hollow ceramic shells encapsulating a controlled-pressure gas that can be ionized to controlled plasma parameters. Plasma-shells are electrically excited by frequency selective surfaces that are transparent to the frequency band of interest. The result is equivalent to large-area free-space plasma confined in a discrete plasma slab. A novel structure is designed with the aid of full-wave simulation and fabricated as a 76.2-mm square array, and transmission performance is tested across different drive voltages and angles of incidence. Switchable attenuation of 7 dB is measured across the passband when driven with 1400 Vpp at 1 MHz. Plasma parameters are estimated from theory and full-wave simulation, with electron density estimated to be 3.6×1012 cm-3. The proposed structure has potential for use on mobile platforms.