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The frequency-response masking (FRM) method allows the design of selective prototype filters for cosine-modulated filter banks (CMFB) with a reduced number of distinct coefficients. Such a methodology may result in filter banks with a large number of bands (e.g. 1024 or more) and a simplified optimization procedure, as there are less parameters to adjust. This work introduces a numerically efficient optimization procedure, based on a quasi-Newton algorithm, for designing selective FRM-based CMFB. The proposed method uses a perfect-reconstruction FRM prototype filter as a starting point and updates the number of bands of the filter bank during the optimization procedure. Examples provided indicate that figures-of-merit, such as intersymbol and intercarrier interference, for the optimized FRM-CMFB structure are significantly improved without increasing the complexity of the resulting structure.