Hi Ken
I’ve just Googled the Fiamma Vent 50. There’s a fair bit of information there that might help you. I hope you find the right information there, or try a local caravan repair place.
Looking at the above photo and as a quick fix, try and put a piece gaffa tape on the inside or the outside for that matter, and then cover the outside area with a weatherproof sealant, moulding it into the required curvature. It might be a quick fix until you can source a more permanent solution.
Cheers from Pete.