What a coincidence to have 2 loose pieces of metal at the same time, and a blessing that alerted you to the underlug problem.
There are 4 flange musket caps that are mandated by most reenacting groups. The wider flanges on the 4 flange caps won't break off to cause injury as easily as the 6 flange caps can.
And they are reported to work just as well as the 6 flange caps.
If you can find some then buy them and give them a try.
Sorry about the copper shard to the forehead.
Recycle! Pure copper scrap is worth about $3.50/lb.!