You can now print your own (if you have a printer)
https://www.thingiverse.com/thing:4256474
The designer says: "Since the block piers have individual differences, I prepared models with different inner dimensions."
The download has files for three caps with slightly different geometry. I suppose you have to experiment to see which work best with your block piers, as explained in the video.

[video=youtube]https://www.youtube.com/watch?v=gj0-X7V2K84[/video]