Fitting 3 grandchildren into a Nissan Juke?

Hello! Are you the present or past owner of a Nissan Juke? If so perhaps you can answer my query:
I am in the UK have three grandchildren - I position the twins` car seats at each side of the back seat of the Juke as they are aged two and always seem to fight in the car. On a booster cushion in the middle sits their sister aged 7. In my previous car I managed to buy an extender clip to fit onto the adult belt to make it easier for her to clip herself in and out. My new car is a Nissan Juke but I am unable to source the extender clip for the middle belt. I have tried various websites both European and American that advertise the extender clips but none fit this middle seat belt, as it has a different mechanism from the two side seat belts (a bit like fitting the wrong key into a keyhole). I have written to Nissan direct but rather frustratingly they merely tell me to go to my nearest Dealership and I`ve tried lots of Dealerships that sell the accessories for the Juke without success. Any advise would be greatly appreciated.