The Attendance Wall was done at the 2004 Smofcon with great success. A couple of walls of a room were covered with paper and columns labeled with each Smofcon: year and location. People were invited to write their name at any Smofcon they had attended.
Two possibilities, depending on space: All Worldcons, or just UK Worldcons.
With any kind of luck the Intersection list will be huge and will require additional paper.
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