I might try fashioning a plate which sits on top of the left dropout, made of spring steel, with a tough plastic layer between the plate and the dropout to prevent wear... and bolt through a shaped anchoring steel projection to sit inside the open part of the dropout... which will prevent the plate turning, then add another layer of spring steel with the shape of the hitch cut out, which will prevent the hitch turning, then add a final layer of spring steel to hold that on, and nut & bolt through, to secure using my QR axle. Of course it would be better if the preceding was one solid piece: 3D print and cast? I'll see what damage a summer of this hitch turning against the steel embedded into that aluminium dropout has done first. Wear prevention measures might not be necessary, if the Dahon's dropout's embedded steel, is much harder than the steel of the hitch.
