The only problem is that the OP built a single wheel trailer. It needs 2 attachment points on either side of the bike frame. You want the attachment points rather spread apart so the leverage isn't concentrated which could allow the trailer to twist out of line with the rest of the bike. The OP mentioned that he wanted to attach the trailer to the seat stay. The more I think about it, the worse of an idea that would be for a single wheel locked frame trailer to use a seat stay or seat post as a lock point to keep the trailer lined up with the rest of the bike.

You want to have a rather wide attachment point for the trailer frame. That way it spreads the leverage across a wide point and doesn't concentrate it all at one point. The seat stay is only about 34mm wide. The rear drop outs are about 135-150mm. Best to attach the frame to the drop out area.

If the trailer was a 2 wheel trailer, then you would only need one hitch attachment point since there would not be any appreciable side loading. You could then use the seat post as the attachment or even the seat stay.