Payment channel networks (PCNs), also known as off-chain networks, implement a common approach to deal with the scalability problem of blockchain networks. They enable users to execute payments without committing them to the blockchain by relying on predefined payment channels. A pair of users can employ a payment even without a direct channel between them, by routing the payment via payment channels involving other intermediate users. Users, together with the channels, form a graph known as the off-chain network topology. The off-chain topology and the payment characteristics affect network performance such as the average number of intermediate users a payment is routed through or the values of transaction fees. In this paper, we study two basic problems in payment channel network design. First, efficiently mapping users to an off-chain topology with a known structure. Second, constructing a topology with a bounded number of channels that can serve users well with associated payments. We design algorithms for both problems while considering several fundamental topologies. We study topology-related real data statistics of Raiden, the off-chain extension for Ethereum as well as of Lightning, the equivalent off-chain layer of Bitcoin. We conduct experiments to demonstrate the effectiveness of the algorithms for these networks.