This paper contributes to the area participatory design where co-writing is treated as a participatory prototyping method. The aim is to provide a method tool that designers can use to increase reflexivity in transboundary projects, giving practitioners a formal voice when reflecting upon the project retrospectively. The concept boundary object (Star & Griesemer, 1989), is in this study translated into a conversation tool that is used to jointly discuss a participatory design project in the context of Global North-South collaboration. The case study describes how the collaboration unfolded involving multiple and diverse actors. The conversation occurs between a Swedish design researcher and a Kenyan Jua Kali practitioner, who are the key authors of this paper. The tool was developed as part of the co-writing process and is based on the online conversation between the two participants. We argue the tool provides space and scaffolding for joint reflection, making it possible to talk about cooperative complexities in a structured yet open way. It encourages participation and creates co-ownership since the project participants can bring forward their different voices and opinions. Since the tool is replicable and adjustable, it can be used in any transboundary project, thus implementing SDG 17 in practice.