<p>An aircraft consists of various electromagnetic cavities including air intake duct, cockpit, radome, inlet and exhaust of heat exchangers, passage for engine bay/other bay for cooling of aircraft systems, etc. These cavities are the prime contributors for aircraft radar cross-section (RCS). The RCS reductions of these cavities are very important to achieve a low observable aircraft configuration. In this paper, a thin-flexible resistive frequency selective surface (FSS)-based radar absorbing material (RAM) of thickness 0.125&#xa0;mm is presented. This thin-flexible RAM is integrated with glass fabric-based pre-preg for obtaining radar absorbing structure (RAS) with an overall thickness of 2.0&#xa0;mm. Computational result shows &gt; 10&#xa0;dB RCS reduction from 12.5 to 40&#xa0;GHz on a flat RAS panel with metal backing. The results are further validated with experiments from 6 to 18&#xa0;GHz of frequencies. This FSS-based RAS is then computationally implemented on different circular cavities with varying L/D ratios and its performance are compared with the conventional non-RAS cavities. Significant RCS reduction demonstrates possible application of FSS-based RAS on various cavities of futuristic aircraft platform for achieving very low observability. Lastly, the performance of proposed RAS with thickness of 2.0&#xa0;mm is also demonstrated on an open-source CAD model of conventional fighter aircraft. Aircraft frontal sector of 0⁰ to 15⁰ shows ~ 7.75&#xa0;dB median RCS reduction and 0⁰ to 60⁰ shows ~ 8.72&#xa0;dB median RCS reduction from 8 to 18&#xa0;GHz frequencies using the proposed FSS-RAM-based RAS of 2.0&#xa0;mm thickness.</p>