Dont know why I said stainless…I actually used an aluminium one from Viper performance. I went for the 16mm barbed version and it wasnt quite the tight fit that I would have liked. However, it’s been there now for a couple of years with no leaks. Sewin’s suggestion of 18mm might be better but not easy to find.
Is this what you're looking for ?
www.ebay.co.uk/itm/184627867597