Magnetic resonance coupling (MRC) has been utilized in wireless power transfer (WPT) systems to achieve mid-range contactless power supply. For many cases, power security is an important issue, say when power transfer to certain users is unintended and therefore needs to be restricted. Moreover, manufacturing costs and physical dimensions are also important issues for mass production. In this paper, we investigate an MRC-WPT system with multiple transmitters, one intended receiver and one unintended receiver. We formulate the problem of maximizing power of intended receiver by designing coil winding numbers of inductors in MRC circuits. Meanwhile, the proposed design should satisfy the total power and manufacturing cost constraints, as well as the power security demands. Although the problem is non-convex, the global optimal solution can still be efficiently obtained via semidefinite programming approach. Simulation results are provided to show the performance of the proposed design.