In the first instance, I would ask one or more of those mobile body repair outfits for their opinion and quote. It's amazing what can be invisibly repaired these days. I believe that they often do minor repairs, both inside and out for dealers.
Failing that it's a new or second hand trim panel. I believe that there is a company in the West Country that breaks Calis and does conversions. Kernow Campers rings a bell but I'm sure others on the forum will know for sure and give the full information about said company. Might be worth contacting them as they may have a better second hand panel.