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Good on Ya!
I also sometimes use the Dremel mandrel with the very small screw ( less than 1/16th") as an arbor to make the washers round or any other doming or modification to the outer diameter. A...

Everything I try takes a few tries to get it correct! I learn, usually, from the trials.
Have fun!
~Richard

Some guys make their own. Take a small ball pein hammer hit it with a bigger one and drive a stainless washer into a block of lead.
~Richard