I made 3 plugs.

1) UHMWPE
2) Brass, free machining 360
3) Nylon

The UHMWPE was way too soft.

Brass worked best, giving a slightly larger indentation with each shot although indentation slowed as the dent got bigger.

Nylon was right behind the brass, but looks like the indentation was getting bigger a bit faster.

I tested the brass and nylon 15 shots each, and they look like both have many more shots left in them.
You can tell that they are working because the firing pin has a muffled sound to it.
As the dent grows, especially the brass, you must scrape away the raised crater rim.