'87 Firebird wheel centre cap question

I have an '87 Firebird which I bought with a couple of the plastic wheel centre caps missing. I bought a set of GM replacements from Ebay but I'm not sure how they are supposed to be fitted.

One of my old caps had a metal ring behind it that seemed to clip it to the wheel, the other didn't. The new set don't have any metal clips supplied, and when I tried one on a wheel it was very loose, to the point that I was worried it would come off at the first pothole.

Can someone tell me how they are supposed to be fitted?

Thanks

I had the same problem, you have to buy the metal ring with the clips built in, or like my caps, they have the ring melted into the plastic, from GM that way, yours have to be after market made to replace a cracked one where the metal ring still exists. sorry