I saw a pretty cool trailer behind a motorcycle one day. It had ONE WHEEL on it! I want to build one now. I think the idea is that it's more streamlined and doesn't have as much drag as a trailer with two wheels. I'm thinking to attach it, use a CV joint like what you'd find on the driveshaft or half-shaft of a car. That way, the trailer can move up, down, and side-to-side and it will stay parallel to the moped and lean into the turns with the moped.