Z-complementary sequence sets (ZCSSs) can be applied to multi-carrier code division multiple access (MC-CDMA) systems as user address codes to eliminate multi-path interference and multi-access interference. In this paper, two constructions of ZCSSs are investigated. First of all, flexible and diverse seed sequence sets and orthogonal matrices are employed to generate a large number of optimal ZCSSs, which can enrich the existing results. In fact, most ZCSSs lengths obtained by existing orthogonal-matrix-based constructions are required to be divisible by ZCZ widths. Obviously, it cannot be achieved if the desired sequence length is a prime number or a non-integer multiple of the ZCZ width. Toward the challenge of designing ZCSSs of various lengths, direct construction is presented in this paper using orthogonal matrices. Thus, it could provide a variety of quality of service (QoS) in quasi-synchronous multiuser multi-rate communication systems. Meanwhile, the flock sizes can be chosen flexibly, which means that the MC-CDMA systems are no longer limited by sub-carriers. To the best of our knowledge, the construction of ZCSSs have not been reported in the literature. In this paper, the proposed two constructions are direct without the aid of any special sequence, which is suitable for rapid hardware generation.