While popular digital maps support an unprecedented number of use cases, new reference map customization tools have been created for purposes for which those maps fall short. With the goal of informing the design of this new class of cartographic tools, we present the first study of naturalistic (" bespoke ") map customization behavior. Through a mixed methods and mixed-media approach involving a survey, the analysis of a corpus of customized maps, and an interview with a power user, we find that bespoke map customization is a relatively common activity and identify frequent use cases as well as map customization strategies. We discuss these use cases and strategies in detail, and propose design implications for future customization tools, such as the use of templates for common use cases, adaptability for various customization styles and the support of multimedia interaction.