I searched for something strong (and large) enough yesterday and found these ones made by Timco - they are chrome plated brass (also made in other finishes and sizes - 12mm,14mm,16mm 18mm) and the caps screw on. I will use a little thread lock to ensure they don't depart earlier than I want! I actually bought these ones from their company shop on Amazon, so they arrived today via Prime. Cost £5.81.
I know they're not domed like the originals, however they're also not plastic and likely to go brittle and collapse (like the one it's replacing!). Being brass they should last well too. Having found 16mm to be the right size (by chance they were the only ones in stock for immediate dispatch!) I have now ordered some more in satin nickel finish, which should look better with the silver spats.