Martyn said:
. . . . If the coupler is not SAE J684 certified it can't legally be used on the road in the USA or Canada. . . .
This blanket statement is inaccurate.
In most states the SAE J684 certification requirement applies
only to commercial manufacturers and other entities in the stream of commerce, not to the use of home built couplers by a private party. Thus, Adventure Trailers, Kilby (Max Coupler), and Lock N Roll would need certification of the couplers they market, but someone building a coupler for their own trailer and their own use would not. In the event of an accident involving such a home built coupler without certification, that fact alone would not warrant a citation and a plaintiff's lawyer would have to prove with expert testimony not only that the hitch design and construction was dangerous and/or defective, but also that the defective/dangerous hitch design/construction contributed to the damages alleged. It would not be enough just to argue the lack of SAE J684 certification.
That being said, it is tough to argue with the wisdom of always using a product that has passed industry certification requirements if for no other reason than peace of mind.