A federated Byzantine agreement (FBA) system is a permissionless system in which each participant declares unilateral agreement requirements that collectively determine a set of quorums. The resulting quorums can be used in a consensus algorithm (such as the Stellar Consensus Protocol) to build a permissionless blockchain system without resorting to proof-of-work or proof-of-stake. Like most permissionless systems, FBA systems must rely on a secure peer-to-peer overlay network for communication between network nodes, yet this topic has received little attention in the FBA setting. In this paper, we address the problem of connecting the nodes in an FBA system to each other in order to construct an overlay topology that securely and efficiently supports gossip protocols, a popular class of protocols for disseminating data across blockchain overlays. We present Constellation, an algorithm that computes an overlay topology whose fault tolerance matches that of the FBA quorum system. Constellation minimizes node degree, which reduces redundant traffic in gossip protocols, and it ensures a maximum diameter of 2, which is crucial for achieving low-latency consensus.