Stabalizer bracket for detatched swan neck towball

I'm trying to find out where to get a stabalizer bracket for a swan neck towball. We have a bulldog/Q2000 flat bar stabaliser type. The problem being is the detatchable swan neck towball is not straight but is tapered and so everytime we fit a bracket to it, it slips off and then slides up and down.  Using a 51 registered ford galaxy 2.5 petrol manual and a lunar silver jubilee 4 berth. Sold a bracket from towsure which is the right one as long as you dont have a tapered swan neck towball. We have tried a alco hitch but it dosent fit the detatchable ball, got one second hand to use but despite removing all the grease the ball is to big to fit. I've tried looking everywhere to find a suitable bracket but it seems no-one does one. We've been given idea's to either drill a small hole into the swan neck for the grub screw to tighten into or to weld the bracket onto the swan neck. Both of which is ILLEGAL and so is any alterations made to the original tow equipment on the vehicle and now is part of the mot test. Any welding could cause changes to the metal structure and cause a failure. At the moment its towing without a stabaliser. Any ideas?