A new flexible design concept of dual-band RASs is presented and discussed. The new design concept can provide a simple and useful fabrication method. A case study demonstrated the ability of the proposed approach to overcome the limits encountered in previous studies related to the finite design of RASs. To obtain various optimal RAS design solutions using lossy materials, Glass/CNT-EP composites whose permittivity was controlled simply by varying the manufacturing pressure were demonstrated. The feasibility of RASs based on these composite materials with adjustable permittivity was studied at both single- and dual-band frequencies. The structures demonstrated enhanced performance compared to conventional materials. Various design solutions using both single- and double-slab absorbers were obtained on the basis of their properties. Thus, the proposed design method based on the controllable characteristics of CNT-added composite materials is used to enhance the absorption performance of RASs through a flexible design at both single- and dual-band frequencies.