OK, I know my trailer has an "over the wheel" drawbar:





I use a 10 mm diameter quick attach industrial ball joint for the trailer hook up, these are reasonably priced and readily available from Engineering supply companies.

http://www.springfix.co.uk/sectionpd...her-joints.pdf type R3535

There is absolutely no reason why these ball joints can not be fixed to the bicycle's chainstay by using a piece of angle iron cut and shaped to suit with hand tools and fixed with suitably sized U bolts.

Tomorrow I will make a chainstay bracket to use with a ball joint and photograph the process to show you exactly what I mean.