This paper proposes a simple, but efficient solution to avoid a potential problem for filtered OFDM (F-OFDM) that may violate regular resource grid structure. F-OFDM is an extension of OFDM by additional per-subband filtering and a well-localized filter can be utilized to significantly lower the out-of-band radiation that can mitigate interference in asynchronous access. Even with the filtering, additional guard bands that carry no data may be needed between the subbands for different users to keep the interference level sufficiently low. The required guard bandwidth may vary since various aspects should be taken into account such as interference power, modulation format or quality of service requirement. If the subbands are simply shifted to add the guard bands, the regular resource grid will be destroyed. One option to keep the regular resource grid while supporting flexible guard bands is to define several filters, each of which is optimized for each possible guard bandwidth. But this is rather complex. We propose a much simpler solution allocating the guard bands inside the filter bandwidth, which is kept independent of guard bandwidths. We show through detailed analytical and numerical analysis that the proposed solution performs even better than the more complex one in practically relevant interference scenarios.