Firm called BakRak do a swan-neck adapter, it clamps onto the ball (very clever patented clam-shell device, locks with a single bolt and is tested to 600KG limit

so will easily take the load ) With this adapter you can mount any normal 2-bolt towball fitting to it and it then works as it`s supposed to. I`ve got one on my Westy swan-neck towbar, along with a Dave Cooper scooter rack mounted to it, works fine and still comes in under the towbar max load limit of 100KG
www.bak-rak.com
Look for the swan-neck convertor, costs about £30, ring `em up and order, as their online shop doesn`t work properly, which I`ve told `em about in numerous phone calls

Took me a few calls to get one delivered, don`t know why, they don`t seem right well organised, but the product is the dogs

Keep it real.
Search first - ask second ;>}