Cannondale Compression Plug, O Ring?

Hi, I'm having a bit of aggro with my Topstone Alloy. All self-inflicted too!

I loosened the top cap but actually it was the expander plug below I was opening and all these bits in the photo dropped into the tube. I think I got all 4 pieces out but looking at the plugs on YouTube there seems to be maybe a rubber o-ring holding them to the plug.

Is this just a pretty general type o-ring? I can't find the original that was there at all now. Reckon I will get buy with some cheap replace ring that's a good diameter?

Thanks!