Lost Diesel Cap for Hymer B544

Romatone

Filled up recently ahead of my winter travels and a day later realised I had left my cap behind. Went back and no one had handed it in. Has anyone else done the same  thing , probably not. As its a Fiat Chassis I assumed it was a Fiat cap but sadly I was
wrong. As the one ordered online arrived today and  doesn't fit. I guess I can get a replacement from Hymer but was looking at an alternative first ( thinking about the price) any help appreciated, thank you