Overlay multicast protocols construct a virtual mesh spanning all member nodes of a multicast group. It employs standard unicast routing and forwarding to fulfill multicast functionality. The advantages of this approach are robustness and low overhead. However, efficiency and stability are the issues that must be addressed in the mobile ad hoc network (MANET) environment. In this paper, we propose an effective structure for overlay multicast to solve these problems in MANET. Instead of using a spanning tree on the virtual mesh, we introduce a simple structure called k-tree trunk for multicast. A k-tree trunk of a tree is a subtree with k leaves that minimizes the sum of the distances of all vertices to the subtree plus the size of the subtree. The k-tree trunk is more stable and easier to maintain than the spanning tree in MANET. The simulation results show that our approach handles the flexibility and mobility issues in an overlay multicast protocol effectively, especially when the group size is large.