Maker education provides the perfect context for young learners to develop 21st-century skills. However, research is inconclusive on how these skills could be assessed. Namely, the complex nature of 21st-century skills requires different types of assessments, not necessarily relying on paper-and-pencil or multiple-choice tests, but rather drawing on the learners' perspective in the form of self-assessment and reflection. Prior studies highlighted several challenges of situating self-assessment in makerspace contexts, such as the lack of dedicated technology for documentation, distractions caused by noise or group work, and the lack of skills and motivation to practice self-assessment. This paper presents an exploratory case study aimed at an in-depth investigation of the use of a digital self-assessment tool of 21st-century skills in makerspace contexts. The authors converged qualitative data collected mainly from interviews with teachers and students. Researcher observations and tool log files (e.g., student work in the digital tool) were used as triangulation sources. Although challenges emerged, the study presents encouraging findings regarding the use of the digital tool for raising students' awareness of their development of 21st-century skills and engaging them in self-assessment and reflection. The results of the study provide rich insights to guide future research on the topic.