I have a brass one of these I made a while back and it works quite well.
I wanted something that would work for aluminum parts without being
hard enough to dent it. Here is the brass one:
I used aluminum for the actual knocker but I bored it out and filled it with lead.
Feels just about right. While I was at it I made a short one out of a piece
of aluminum scrap. I prefer the longer type and the way it is made.
You can hit with the lead end or from the aluminum end which keeps the lead
off your aluminum and brass parts.
The short one is good to make sure you are doing the hitting with aluminum
and not the lead since it was loaded from the side.
Read Understand and Follow all the warnings when using and working with and melting LEAD.