The correct washers for these were difficult for me to find. They're 1/2 inch OD x 1/8 inch ID aluminum. The rivets I used are 1/16in - 1/8in grip range. I always thought the washers on this part looked like the sides were filed down. They definitely are or at least that's what I believe. There's no way the washers fit in that space otherwise.
The washer in this picture is the correct outside diameter but the inside diameter needs to be 1/8 inch. This is just an example to show it doesn't fit unless you file down the sides.
The washer in this picture is the correct outside diameter but the inside diameter needs to be 1/8 inch. This is just an example to show it doesn't fit unless you file down the sides.