Using some .063 thick scrap, I fabricated these two washers. The holes were just drilled up to 7/16 with a unibit, and I sanded down the OD by hand. Not too bad, but I'd like to make the OD smaller and more concentric.

After a bit of head scratching, this is what I came up with. I used one of the 3/8 inch NIB magnets my brother, Chris, gave me to create kind of a post on my sander. Then I just sanded away the opposite side of the washer until the ID contacted the post. Rotating the washer around gave a nice, concentric OD.

I was also able to make the OD somewhat smaller, so the edges of the washers don't overhang the bracket.

Here they are temporarily installed. A little primer and these'll be good to go.