rear middle seat belt .. defender 110 DCab .. childrens car seat!

hi .. am about to have a 3rd small person to cart around ..
need to put a 3rd car seat in back of my beloved 110 double cab .. problem is, there is only a lap belt ..
has anyone converted, or know how to convert it to take a normal inertia shoulder belt so that I can legally carry the kids ? .. many thanks for your help!
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Puma rear seat conversion it has 3 seats with proper belts