Many community open source projects are of high economic relevance. As these projects mature, their leaders face a choice of continuing the project as is, making the project join an existing foundation, or creating their own foundation for the project. This article presents a model of open source developer foundations that project leaders can use to compare existing foundations with their needs or to design their own. The model is based on a three-iteration qualitative study involving interviews and supplementary materials review. To demonstrate its usefulness, we apply the model to nine foundations and present their organizational choices in a comparative table format.