My co-designer is presenting our game map as his solo project. How direct should I be?

The club showcase slide said "Map by Owen." I had built the underground route, tested the spawn points with him, and spent two weekends fixing places where players got stuck.

Owen said he'd used his name because he was submitting the map and answering questions at the event. He isn't denying I helped; he offered to mention me when somebody asks about the tunnels. But most people will see the slide and play the map without asking.

We never discussed credits because it was a hobby project between friends. I don't want to pretend every contribution was equal: he made the overall layout and did most of the visual work. I do want more than being a footnote he remembers if a question happens to come up.

The showcase is next Friday, and the slide can still be edited. What credit wording would be fair without turning this into a contest over which of us worked more hours?