They're just a one-piece lump of plastic (plus a spring). You can't repair them in any way.
I've had one missing for about a year, plus another that's clinging on by its fingernails.
I'd assume they cost a fortune from Fiat. I was looking at some blank ones (no badge at all) on ebay once that were the right size.
If you want Fiat ones then I'd bet that they'll be a different size to any other Fiat, due to the Vauxhall heritage. I'd hope to be wrong though.
Edited to add: The silicone idea is a very good one. Except that I wonder if there's a risk of trapping moisture - in a cavity, on the end of the axle? As far as I know, there's no good reason to ever remove them anyway.