This splitter did the trick after I wedged it in place with a chunk of wood to stop the ball bearing scooting off the prong every time I tightened the bolt.
The chunky bolt enlarges the hole in the rubbers slightly but they should be ok.
I've only unhitched the Bosal prongs for the moment - I decided to leave the rubbers on the car's prongs because I won't be able to reach them once the Klarius box is in position (it's much wider than the Bosal back box).
Anyway, a top tip from Arctic!