Background: Open Source Software (OSS) fuels our global digital infrastructure but is commonly maintained by small groups of people whose time and labor represent a depletable resource. For the OSS projects to stay sustainable, i.e., viable and maintained over time without interruption or weakening, maintenance labor requires an underlying infrastructure to be supported and secured. Aims: Using the construct of human infrastructure, our study aims to investigate how maintenance labor can be supported and secured to enable the creation and maintenance of sustainable OSS projects, viewed from the maintainers' perspective. Method: In our exploration, we interviewed ten maintainers from nine well-adopted OSS projects. We coded the data in two steps using investigator-triangulation. Results: We constructed a framework of infrastructure design that provide insight for OSS projects in the design of their human infrastructure. The framework specifically highlight the importance of human factors, e.g., securing a work-life balance and proactively managing social pressure, toxicity, and diversity. We also note both differences and overlaps in how the infrastructure needs to support and secure maintenance labor from maintainers and the wider OSS community, respectively. Funding is specifically highlighted as an important enabler for both types of resources. Conclusions: The study contributes to the qualitative understanding of the importance, sensitivity, and risk for depletion of the maintenance labor required to build and maintain healthy OSS projects. Human infrastructure is pivotal in ensuring that maintenance labor is sustainable, and by extension the OSS projects on which we all depend.