Do you have a welder? I think i am going to research my hookup to mount on the rear axle rather then the seat post. Im thinking of a hitch reciver wraped around the rear tire with a U-joint mounted to the trailer. Most people do a straight shot to the seat post but im wondering if having the mount lower would benefit better then the torque on the seat post. On youtube there is a guy who made a trailer setup with a u-joint. You might want to serach for it.
http://www.youtube.com/watch?v=xq20jjZSYRg