Any suggestions for center caps?

[Tutosnake]

When I switched to FIA wheels, the "original" center caps that came with the Halis sank in at the rear. I do not like this look:


So, I removed the rear caps and although I like the "unadorned" racy look, I do not want to get stuck cleaning down in there:



I then stuck a yellow ball down the opening. I couldn't find the right size ball, so I used a double strip of shipping tape around each ball, and this help fill in the small gap:



Any suggestions? I would like to find something that will stick in the hole without unreliable tape holding it. I drove the car for an hour or so and the balls didn't come off (and I am NOT talking about my own).
Anyone has done something different?

Hector